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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Thu, 30 Mar 2023 12:31:22 -0700
Message-ID: <CAJuCfpEZu9qAm+mWRZRuCFy7yQfpwz0W4N=5Fd1msxWG55yHXg@mail.gmail.com>
Subject: Re: [PATCH v6 4/4] sched/psi: allow unprivileged polling of N*2s period
To:     Domenico Cerasuolo <cerasuolodomenico@gmail.com>
Cc:     linux-kernel@vger.kernel.org, peterz@infradead.org,
        brauner@kernel.org, chris@chrisdown.name, hannes@cmpxchg.org
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On Thu, Mar 30, 2023 at 12:06=E2=80=AFPM Domenico Cerasuolo
<cerasuolodomenico@gmail.com> wrote:
>
>
>
> On Thu, Mar 30, 2023 at 8:51=E2=80=AFPM Suren Baghdasaryan <surenb@google=
.com> wrote:
>>
>> On Thu, Mar 30, 2023 at 3:54=E2=80=AFAM Domenico Cerasuolo
>> <cerasuolodomenico@gmail.com> wrote:
>> >
>> > PSI offers 2 mechanisms to get information about a specific resource
>> > pressure. One is reading from /proc/pressure/<resource>, which gives
>> > average pressures aggregated every 2s. The other is creating a pollabl=
e
>> > fd for a specific resource and cgroup.
>> >
>> > The trigger creation requires CAP_SYS_RESOURCE, and gives the
>> > possibility to pick specific time window and threshold, spawing an RT
>> > thread to aggregate the data.
>> >
>> > Systemd would like to provide containers the option to monitor pressur=
e
>> > on their own cgroup and sub-cgroups. For example, if systemd launches =
a
>> > container that itself then launches services, the container should hav=
e
>> > the ability to poll() for pressure in individual services. But neither
>> > the container nor the services are privileged.
>> >
>> > This patch implements a mechanism to allow unprivileged users to creat=
e
>> > pressure triggers. The difference with privileged triggers creation is
>> > that unprivileged ones must have a time window that's a multiple of 2s=
.
>> > This is so that we can avoid unrestricted spawning of rt threads, and
>> > use instead the same aggregation mechanism done for the averages, whic=
h
>> > runs independently of any triggers.
>> >
>> > Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
>> > Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
>> > Acked-by: Johannes Weiner <hannes@cmpxchg.org>
>> > ---
>> >  Documentation/accounting/psi.rst |   4 +
>> >  include/linux/psi.h              |   2 +-
>> >  include/linux/psi_types.h        |   7 ++
>> >  kernel/cgroup/cgroup.c           |   2 +-
>> >  kernel/sched/psi.c               | 175 +++++++++++++++++++-----------=
-
>> >  5 files changed, 121 insertions(+), 69 deletions(-)
>> >
>> > diff --git a/Documentation/accounting/psi.rst b/Documentation/accounti=
ng/psi.rst
>> > index 5e40b3f437f9..df6062eb3abb 100644
>> > --- a/Documentation/accounting/psi.rst
>> > +++ b/Documentation/accounting/psi.rst
>> > @@ -105,6 +105,10 @@ prevent overly frequent polling. Max limit is cho=
sen as a high enough number
>> >  after which monitors are most likely not needed and psi averages can =
be used
>> >  instead.
>> >
>> > +Unprivileged users can also create monitors, with the only limitation=
 that the
>> > +window size must be a multiple of 2s, in order to prevent excessive r=
esource
>> > +usage.
>> > +
>> >  When activated, psi monitor stays active for at least the duration of=
 one
>> >  tracking window to avoid repeated activations/deactivations when syst=
em is
>> >  bouncing in and out of the stall state.
>> > diff --git a/include/linux/psi.h b/include/linux/psi.h
>> > index b029a847def1..ab26200c2803 100644
>> > --- a/include/linux/psi.h
>> > +++ b/include/linux/psi.h
>> > @@ -24,7 +24,7 @@ void psi_memstall_leave(unsigned long *flags);
>> >
>> >  int psi_show(struct seq_file *s, struct psi_group *group, enum psi_re=
s res);
>> >  struct psi_trigger *psi_trigger_create(struct psi_group *group,
>> > -                       char *buf, enum psi_res res);
>> > +                       char *buf, enum psi_res res, struct file *file=
);
>> >  void psi_trigger_destroy(struct psi_trigger *t);
>> >
>> >  __poll_t psi_trigger_poll(void **trigger_ptr, struct file *file,
>> > diff --git a/include/linux/psi_types.h b/include/linux/psi_types.h
>> > index 1819afa8b198..040c089581c6 100644
>> > --- a/include/linux/psi_types.h
>> > +++ b/include/linux/psi_types.h
>> > @@ -151,6 +151,9 @@ struct psi_trigger {
>> >
>> >         /* Deferred event(s) from previous ratelimit window */
>> >         bool pending_event;
>> > +
>> > +       /* Trigger type - PSI_AVGS for unprivileged, PSI_POLL for RT *=
/
>> > +       enum psi_aggregators aggregator;
>> >  };
>> >
>> >  struct psi_group {
>> > @@ -171,6 +174,10 @@ struct psi_group {
>> >         /* Aggregator work control */
>> >         struct delayed_work avgs_work;
>> >
>> > +       /* Unprivileged triggers against N*PSI_FREQ windows */
>> > +       struct list_head avg_triggers;
>> > +       u32 avg_nr_triggers[NR_PSI_STATES - 1];
>> > +
>> >         /* Total stall times and sampled pressure averages */
>> >         u64 total[NR_PSI_AGGREGATORS][NR_PSI_STATES - 1];
>> >         unsigned long avg[NR_PSI_STATES - 1][3];
>> > diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
>> > index 935e8121b21e..dead36969bba 100644
>> > --- a/kernel/cgroup/cgroup.c
>> > +++ b/kernel/cgroup/cgroup.c
>> > @@ -3761,7 +3761,7 @@ static ssize_t pressure_write(struct kernfs_open=
_file *of, char *buf,
>> >         }
>> >
>> >         psi =3D cgroup_psi(cgrp);
>> > -       new =3D psi_trigger_create(psi, buf, res);
>> > +       new =3D psi_trigger_create(psi, buf, res, of->file);
>> >         if (IS_ERR(new)) {
>> >                 cgroup_put(cgrp);
>> >                 return PTR_ERR(new);
>> > diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
>> > index f3df6a8ff493..e072f6b31bf3 100644
>> > --- a/kernel/sched/psi.c
>> > +++ b/kernel/sched/psi.c
>> > @@ -186,9 +186,14 @@ static void group_init(struct psi_group *group)
>> >                 seqcount_init(&per_cpu_ptr(group->pcpu, cpu)->seq);
>> >         group->avg_last_update =3D sched_clock();
>> >         group->avg_next_update =3D group->avg_last_update + psi_period=
;
>> > -       INIT_DELAYED_WORK(&group->avgs_work, psi_avgs_work);
>> >         mutex_init(&group->avgs_lock);
>> > -       /* Init trigger-related members */
>> > +
>> > +       /* Init avg trigger-related members */
>> > +       INIT_LIST_HEAD(&group->avg_triggers);
>> > +       memset(group->avg_nr_triggers, 0, sizeof(group->avg_nr_trigger=
s));
>> > +       INIT_DELAYED_WORK(&group->avgs_work, psi_avgs_work);
>> > +
>> > +       /* Init rtpoll trigger-related members */
>> >         atomic_set(&group->rtpoll_scheduled, 0);
>> >         mutex_init(&group->rtpoll_trigger_lock);
>> >         INIT_LIST_HEAD(&group->rtpoll_triggers);
>> > @@ -430,21 +435,32 @@ static u64 window_update(struct psi_window *win,=
 u64 now, u64 value)
>> >         return growth;
>> >  }
>> >
>> > -static u64 update_triggers(struct psi_group *group, u64 now, bool *up=
date_total)
>> > +static u64 update_triggers(struct psi_group *group, u64 now, bool *up=
date_total,
>> > +                                                  enum psi_aggregator=
s aggregator)
>> >  {
>> >         struct psi_trigger *t;
>> > -       u64 *total =3D group->total[PSI_POLL];
>> > +       u64 *total =3D group->total[aggregator];
>> > +       struct list_head *triggers;
>> > +       u64 *aggregator_total;
>> >         *update_total =3D false;
>> >
>> > +       if (aggregator =3D=3D PSI_AVGS) {
>> > +               triggers =3D &group->avg_triggers;
>> > +               aggregator_total =3D group->avg_total;
>> > +       } else {
>> > +               triggers =3D &group->rtpoll_triggers;
>> > +               aggregator_total =3D group->rtpoll_total;
>> > +       }
>> > +
>> >         /*
>> >          * On subsequent updates, calculate growth deltas and let
>> >          * watchers know when their specified thresholds are exceeded.
>> >          */
>> > -       list_for_each_entry(t, &group->rtpoll_triggers, node) {
>> > +       list_for_each_entry(t, triggers, node) {
>> >                 u64 growth;
>> >                 bool new_stall;
>> >
>> > -               new_stall =3D group->rtpoll_total[t->state] !=3D total=
[t->state];
>> > +               new_stall =3D aggregator_total[t->state] !=3D total[t-=
>state];
>> >
>> >                 /* Check for stall activity or a previous threshold br=
each */
>> >                 if (!new_stall && !t->pending_event)
>> > @@ -546,6 +562,7 @@ static void psi_avgs_work(struct work_struct *work=
)
>> >         struct delayed_work *dwork;
>> >         struct psi_group *group;
>> >         u32 changed_states;
>> > +       bool update_total;
>> >         u64 now;
>> >
>> >         dwork =3D to_delayed_work(work);
>> > @@ -563,8 +580,10 @@ static void psi_avgs_work(struct work_struct *wor=
k)
>> >          * Once restarted, we'll catch up the running averages in one
>> >          * go - see calc_avgs() and missed_periods.
>> >          */
>> > -       if (now >=3D group->avg_next_update)
>> > +       if (now >=3D group->avg_next_update) {
>> > +               update_triggers(group, now, &update_total, PSI_AVGS);
>> >                 group->avg_next_update =3D update_averages(group, now)=
;
>> > +       }
>> >
>> >         if (changed_states & PSI_STATE_RESCHEDULE) {
>> >                 schedule_delayed_work(dwork, nsecs_to_jiffies(
>> > @@ -574,7 +593,7 @@ static void psi_avgs_work(struct work_struct *work=
)
>> >         mutex_unlock(&group->avgs_lock);
>> >  }
>> >
>> > -static void init_triggers(struct psi_group *group, u64 now)
>> > +static void init_rtpoll_triggers(struct psi_group *group, u64 now)
>> >  {
>> >         struct psi_trigger *t;
>> >
>> > @@ -667,7 +686,7 @@ static void psi_rtpoll_work(struct psi_group *grou=
p)
>> >         if (changed_states & group->rtpoll_states) {
>> >                 /* Initialize trigger windows when entering polling mo=
de */
>> >                 if (now > group->rtpoll_until)
>> > -                       init_triggers(group, now);
>> > +                       init_rtpoll_triggers(group, now);
>> >
>> >                 /*
>> >                  * Keep the monitor active for at least the duration o=
f the
>> > @@ -684,7 +703,7 @@ static void psi_rtpoll_work(struct psi_group *grou=
p)
>> >         }
>> >
>> >         if (now >=3D group->rtpoll_next_update) {
>> > -               group->rtpoll_next_update =3D update_triggers(group, n=
ow, &update_total);
>> > +               group->rtpoll_next_update =3D update_triggers(group, n=
ow, &update_total, PSI_POLL);
>> >                 if (update_total)
>> >                         memcpy(group->rtpoll_total, group->total[PSI_P=
OLL],
>> >                                    sizeof(group->rtpoll_total));
>> > @@ -1254,16 +1273,23 @@ int psi_show(struct seq_file *m, struct psi_gr=
oup *group, enum psi_res res)
>> >  }
>> >
>> >  struct psi_trigger *psi_trigger_create(struct psi_group *group,
>> > -                       char *buf, enum psi_res res)
>> > +                       char *buf, enum psi_res res, struct file *file=
)
>> >  {
>> >         struct psi_trigger *t;
>> >         enum psi_states state;
>> >         u32 threshold_us;
>> > +       bool privileged;
>> >         u32 window_us;
>> >
>> >         if (static_branch_likely(&psi_disabled))
>> >                 return ERR_PTR(-EOPNOTSUPP);
>> >
>> > +       /*
>> > +        * Checking the privilege here on file->f_cred implies that a =
privileged user
>> > +        * could open the file and delegate the write to an unprivileg=
ed one.
>> > +        */
>> > +       privileged =3D cap_raised(file->f_cred->cap_effective, CAP_SYS=
_RESOURCE);
>> > +
>> >         if (sscanf(buf, "some %u %u", &threshold_us, &window_us) =3D=
=3D 2)
>> >                 state =3D PSI_IO_SOME + res * 2;
>> >         else if (sscanf(buf, "full %u %u", &threshold_us, &window_us) =
=3D=3D 2)
>> > @@ -1283,6 +1309,13 @@ struct psi_trigger *psi_trigger_create(struct p=
si_group *group,
>> >                 window_us > WINDOW_MAX_US)
>> >                 return ERR_PTR(-EINVAL);
>> >
>> > +       /*
>> > +        * Unprivileged users can only use 2s windows so that averages=
 aggregation
>> > +        * work is used, and no RT threads need to be spawned.
>> > +        */
>> > +       if (!privileged && window_us % 2000000)
>> > +               return ERR_PTR(-EINVAL);
>> > +
>> >         /* Check threshold */
>> >         if (threshold_us =3D=3D 0 || threshold_us > window_us)
>> >                 return ERR_PTR(-EINVAL);
>> > @@ -1302,31 +1335,40 @@ struct psi_trigger *psi_trigger_create(struct =
psi_group *group,
>> >         t->last_event_time =3D 0;
>> >         init_waitqueue_head(&t->event_wait);
>> >         t->pending_event =3D false;
>> > +       t->aggregator =3D privileged ? PSI_POLL : PSI_AVGS;
>> >
>> > -       mutex_lock(&group->rtpoll_trigger_lock);
>> > +       if (privileged) {
>> > +               mutex_lock(&group->rtpoll_trigger_lock);
>> >
>> > -       if (!rcu_access_pointer(group->rtpoll_task)) {
>> > -               struct task_struct *task;
>> > +               if (!rcu_access_pointer(group->rtpoll_task)) {
>> > +                       struct task_struct *task;
>> >
>> > -               task =3D kthread_create(psi_rtpoll_worker, group, "psi=
mon");
>> > -               if (IS_ERR(task)) {
>> > -                       kfree(t);
>> > -                       mutex_unlock(&group->rtpoll_trigger_lock);
>> > -                       return ERR_CAST(task);
>> > +                       task =3D kthread_create(psi_rtpoll_worker, gro=
up, "psimon");
>> > +                       if (IS_ERR(task)) {
>> > +                               kfree(t);
>> > +                               mutex_unlock(&group->rtpoll_trigger_lo=
ck);
>> > +                               return ERR_CAST(task);
>> > +                       }
>> > +                       atomic_set(&group->rtpoll_wakeup, 0);
>> > +                       wake_up_process(task);
>> > +                       rcu_assign_pointer(group->rtpoll_task, task);
>> >                 }
>> > -               atomic_set(&group->rtpoll_wakeup, 0);
>> > -               wake_up_process(task);
>> > -               rcu_assign_pointer(group->rtpoll_task, task);
>> > -       }
>> >
>> > -       list_add(&t->node, &group->rtpoll_triggers);
>> > -       group->rtpoll_min_period =3D min(group->rtpoll_min_period,
>> > -               div_u64(t->win.size, UPDATES_PER_WINDOW));
>> > -       group->rtpoll_nr_triggers[t->state]++;
>> > -       group->rtpoll_states |=3D (1 << t->state);
>> > +               list_add(&t->node, &group->rtpoll_triggers);
>> > +               group->rtpoll_min_period =3D min(group->rtpoll_min_per=
iod,
>> > +                       div_u64(t->win.size, UPDATES_PER_WINDOW));
>> > +               group->rtpoll_nr_triggers[t->state]++;
>> > +               group->rtpoll_states |=3D (1 << t->state);
>> >
>> > -       mutex_unlock(&group->rtpoll_trigger_lock);
>> > +               mutex_unlock(&group->rtpoll_trigger_lock);
>> > +       } else {
>> > +               mutex_lock(&group->avgs_lock);
>> > +
>> > +               list_add(&t->node, &group->avg_triggers);
>> > +               group->avg_nr_triggers[t->state]++;
>> >
>> > +               mutex_unlock(&group->avgs_lock);
>> > +       }
>> >         return t;
>> >  }
>> >
>> > @@ -1350,34 +1392,41 @@ void psi_trigger_destroy(struct psi_trigger *t=
)
>> >          */
>> >         wake_up_pollfree(&t->event_wait);
>> >
>> > -       mutex_lock(&group->rtpoll_trigger_lock);
>> > -
>> > -       if (!list_empty(&t->node)) {
>> > -               struct psi_trigger *tmp;
>> > -               u64 period =3D ULLONG_MAX;
>> > -
>> > -               list_del(&t->node);
>> > -               group->rtpoll_nr_triggers[t->state]--;
>> > -               if (!group->rtpoll_nr_triggers[t->state])
>> > -                       group->rtpoll_states &=3D ~(1 << t->state);
>> > -               /* reset min update period for the remaining triggers =
*/
>> > -               list_for_each_entry(tmp, &group->rtpoll_triggers, node=
)
>> > -                       period =3D min(period, div_u64(tmp->win.size,
>> > -                                       UPDATES_PER_WINDOW));
>> > -               group->rtpoll_min_period =3D period;
>> > -               /* Destroy rtpoll_task when the last trigger is destro=
yed */
>> > -               if (group->rtpoll_states =3D=3D 0) {
>> > -                       group->rtpoll_until =3D 0;
>> > -                       task_to_destroy =3D rcu_dereference_protected(
>> > -                                       group->rtpoll_task,
>> > -                                       lockdep_is_held(&group->rtpoll=
_trigger_lock));
>> > -                       rcu_assign_pointer(group->rtpoll_task, NULL);
>> > -                       del_timer(&group->rtpoll_timer);
>> > +       if (t->aggregator =3D=3D PSI_AVGS) {
>>
>> Just a nit, in psi_trigger_create() you are handling privileged
>> trigger case first and then unprivileged case. In
>> psi_trigger_destroy() the order is opposite. Would be better to have
>> the same order.
>> In both cases I would prefer:
>>
>> if (t->aggregator =3D=3D PSI_AVGS) {
>>   ...
>>   goto out;
>> }
>> // unprivileged case
>> ...
>> out:
>>
>> This would cut down on churn and indents. Not critical from functional
>> point of view, so
>>
>> Acked-by: Suren Baghdasaryan <surenb@google.com>
>>
> No problem, I will push v7 with these changes and thanks for the reviews!
>
> Maybe a dumb question but I'm still not familiar with the coding style he=
re,
> `goto out` for destroy is clear. In create though there's just the return=
 statement
> left after the else branch, should I return in the if branch like this:
>
> if (privileged) {
> ...
> return t;
> }
> ...
> return t;
>
> or should the out label just point to the return?

return looks fine to me. Thanks!

>>
>>
>> > +               mutex_lock(&group->avgs_lock);
>> > +               if (!list_empty(&t->node)) {
>> > +                       list_del(&t->node);
>> > +                       group->avg_nr_triggers[t->state]--;
>> >                 }
>> > +               mutex_unlock(&group->avgs_lock);
>> > +       } else {
>> > +               mutex_lock(&group->rtpoll_trigger_lock);
>> > +               if (!list_empty(&t->node)) {
>> > +                       struct psi_trigger *tmp;
>> > +                       u64 period =3D ULLONG_MAX;
>> > +
>> > +                       list_del(&t->node);
>> > +                       group->rtpoll_nr_triggers[t->state]--;
>> > +                       if (!group->rtpoll_nr_triggers[t->state])
>> > +                               group->rtpoll_states &=3D ~(1 << t->st=
ate);
>> > +                       /* reset min update period for the remaining t=
riggers */
>> > +                       list_for_each_entry(tmp, &group->rtpoll_trigge=
rs, node)
>> > +                               period =3D min(period, div_u64(tmp->wi=
n.size,
>> > +                                               UPDATES_PER_WINDOW));
>> > +                       group->rtpoll_min_period =3D period;
>> > +                       /* Destroy rtpoll_task when the last trigger i=
s destroyed */
>> > +                       if (group->rtpoll_states =3D=3D 0) {
>> > +                               group->rtpoll_until =3D 0;
>> > +                               task_to_destroy =3D rcu_dereference_pr=
otected(
>> > +                                               group->rtpoll_task,
>> > +                                               lockdep_is_held(&group=
->rtpoll_trigger_lock));
>> > +                               rcu_assign_pointer(group->rtpoll_task,=
 NULL);
>> > +                               del_timer(&group->rtpoll_timer);
>> > +                       }
>> > +               }
>> > +               mutex_unlock(&group->rtpoll_trigger_lock);
>> >         }
>> >
>> > -       mutex_unlock(&group->rtpoll_trigger_lock);
>> > -
>> >         /*
>> >          * Wait for psi_schedule_rtpoll_work RCU to complete its read-=
side
>> >          * critical section before destroying the trigger and optional=
ly the
>> > @@ -1437,27 +1486,19 @@ static int psi_cpu_show(struct seq_file *m, vo=
id *v)
>> >         return psi_show(m, &psi_system, PSI_CPU);
>> >  }
>> >
>> > -static int psi_open(struct file *file, int (*psi_show)(struct seq_fil=
e *, void *))
>> > -{
>> > -       if (file->f_mode & FMODE_WRITE && !capable(CAP_SYS_RESOURCE))
>> > -               return -EPERM;
>> > -
>> > -       return single_open(file, psi_show, NULL);
>> > -}
>> > -
>> >  static int psi_io_open(struct inode *inode, struct file *file)
>> >  {
>> > -       return psi_open(file, psi_io_show);
>> > +       return single_open(file, psi_io_show, NULL);
>> >  }
>> >
>> >  static int psi_memory_open(struct inode *inode, struct file *file)
>> >  {
>> > -       return psi_open(file, psi_memory_show);
>> > +       return single_open(file, psi_memory_show, NULL);
>> >  }
>> >
>> >  static int psi_cpu_open(struct inode *inode, struct file *file)
>> >  {
>> > -       return psi_open(file, psi_cpu_show);
>> > +       return single_open(file, psi_cpu_show, NULL);
>> >  }
>> >
>> >  static ssize_t psi_write(struct file *file, const char __user *user_b=
uf,
>> > @@ -1491,7 +1532,7 @@ static ssize_t psi_write(struct file *file, cons=
t char __user *user_buf,
>> >                 return -EBUSY;
>> >         }
>> >
>> > -       new =3D psi_trigger_create(&psi_system, buf, res);
>> > +       new =3D psi_trigger_create(&psi_system, buf, res, file);
>> >         if (IS_ERR(new)) {
>> >                 mutex_unlock(&seq->lock);
>> >                 return PTR_ERR(new);
>> > @@ -1571,7 +1612,7 @@ static int psi_irq_show(struct seq_file *m, void=
 *v)
>> >
>> >  static int psi_irq_open(struct inode *inode, struct file *file)
>> >  {
>> > -       return psi_open(file, psi_irq_show);
>> > +       return single_open(file, psi_irq_show, NULL);
>> >  }
>> >
>> >  static ssize_t psi_irq_write(struct file *file, const char __user *us=
er_buf,
>> > --
>> > 2.34.1
>> >