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Date:   Mon, 27 Jun 2022 21:24:58 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Zhangfei Gao <zhangfei.gao@linaro.org>
Cc:     Neeraj Upadhyay <quic_neeraju@quicinc.com>, frederic@kernel.org,
        josh@joshtriplett.org, rostedt@goodmis.org,
        mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com,
        joel@joelfernandes.org, linux-kernel@vger.kernel.org,
        boqun.feng@gmail.com, urezki@gmail.com,
        shameerali.kolothum.thodi@huawei.com, pbonzini@redhat.com,
        mtosatti@redhat.com, eric.auger@redhat.com,
        chenxiang66@hisilicon.com, maz@kernel.org
Subject: Re: [PATCH] srcu: Reduce blocking agressiveness of expedited grace
 periods further
Message-ID: <20220628042458.GW1790663@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
References: <20220627123706.20187-1-quic_neeraju@quicinc.com>
 <60336a5f-e7ab-48e6-2076-8db4ceb0561a@linaro.org>
 <a3603e76-82b5-529d-d1a9-54f7b5214af8@quicinc.com>
 <584f186a-c2d8-0e47-7e4d-9516822697a8@linaro.org>
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On Tue, Jun 28, 2022 at 12:03:10PM +0800, Zhangfei Gao wrote:
> 
> 
> On 2022/6/28 上午11:22, Neeraj Upadhyay wrote:
> > 
> > 
> > On 6/28/2022 7:44 AM, Zhangfei Gao wrote:
> > > 
> > > 
> > > On 2022/6/27 下午8:37, Neeraj Upadhyay wrote:
> > > > Commit 640a7d37c3f4 ("srcu: Block less aggressively for expedited
> > > > grace periods") highlights a problem where aggressively blocking
> > > > SRCU expedited grace periods, as was introduced in commit
> > > > 282d8998e997 ("srcu: Prevent expedited GPs and blocking readers
> > > > from consuming CPU"), introduces ~2 minutes delay to the overall
> > > > ~3.5 minutes boot time, when starting VMs with "-bios QEMU_EFI.fd"
> > > > cmdline on qemu, which results in very high rate of memslots
> > > > add/remove, which causes > ~6000 synchronize_srcu() calls for
> > > > kvm->srcu SRCU instance.
> > > > 
> > > > Below table captures the experiments done by Zhangfei Gao, Shameer,
> > > > to measure the boottime impact with various values of non-sleeping
> > > > per phase counts, with HZ_250 and preemption enabled:
> > > > 
> > > > +──────────────────────────+────────────────+
> > > > | SRCU_MAX_NODELAY_PHASE   | Boot time (s)  |
> > > > +──────────────────────────+────────────────+
> > > > | 100                      | 30.053         |
> > > > | 150                      | 25.151         |
> > > > | 200                      | 20.704         |
> > > > | 250                      | 15.748         |
> > > > | 500                      | 11.401         |
> > > > | 1000                     | 11.443         |
> > > > | 10000                    | 11.258         |
> > > > | 1000000                  | 11.154         |
> > > > +──────────────────────────+────────────────+
> > > > 
> > > > Analysis on the experiment results showed improved boot time
> > > > with non blocking delays close to one jiffy duration. This
> > > > was also seen when number of per-phase iterations were scaled
> > > > to one jiffy.
> > > > 
> > > > So, this change scales per-grace-period phase number of non-sleeping
> > > > polls, soiuch that, non-sleeping polls are done for one jiffy.
> > > > In addition
> > > > to this, srcu_get_delay() call in srcu_gp_end(), which is used
> > > > to calculate
> > > > the delay used for scheduling callbacks, is replaced with the check for
> > > > expedited grace period. This is done, to schedule cbs for
> > > > completed expedited
> > > > grace periods immediately, which results in improved boot time seen in
> > > > experiments.
> > > > 
> > > > In addition to the changes to default per phase delays, this change
> > > > adds 3 new kernel parameters - srcutree.srcu_max_nodelay,
> > > > srcutree.srcu_max_nodelay_phase, srcutree.srcu_retry_check_delay.
> > > > This allows users to configure the srcu grace period scanning delays,
> > > > depending on their system configuration requirements.
> > > > 
> > > > Signed-off-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
> > > 
> > > Test on arm64
> > > git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git dev
> > > 5.19-rc3
> > > 
> > > arch/arm64/configs/defconfig make defconfig CONFIG_PREEMPTION=y
> > > CONFIG_HZ_250=y
> > > 
> > 
> > If it is possible to try out, do you get similar results with HZ_1000?
> 
> CONFIG_HZ_1000=y
> CONFIG_HZ=1000
> 
> with this patch
> real    0m10.560s
> user    0m3.230s
> sys    0m1.024s
> 
> Revert this patch
> real    0m44.014s
> user    0m3.005s
> sys     0m1.287s
> 
> > 
> > > real    0m11.498s
> > > user    0m2.911s
> > > sys    0m1.171s
> > > 
> > > 
> > > As comparison (Since can not directly revert on linux-rcu.git dev,
> > > so use rc1 instead)
> > > 
> > > 5.19-rc1 + Revert "srcu: Prevent expedited GPs and blocking readers
> > > from consuming CPU"
> > > 
> > > real    0m8.173s
> > > user    0m3.024s
> > > sys    0m0.959s
> > > 
> > > 5.19-rc1
> > > real    2m41.433s
> > >   user    0m3.097s
> > > sys     0m1.177s
> > > 
> > 
> > Thanks! Can I add your Tested-by in subsequent versions of the patch?
> 
> Curiously, is this treated as fixed?
> 11.498s vs. v5.18 8.173s

Please keep in mind that v5.18 was hard-hanging for some people because
of the original behavior of SRCU.  So if you can make this go faster,
so much the better, and I would welcome the patches.  But not at the
expanse of inflicting hard hangs on other people!

Just out of curiosity, what is the total boot time for these QEMU "-bios
QEMU_EFI.fd" runs?

							Thanx, Paul

> Thanks
> 
> > 
> > The numbers are aligned to the initial experiments, without using long
> > retry delays of 100 us. Using long delays might have impact on other
> > workloads, which could be sensitive to the delay between retries. So,
> > I didn't include that in the patch.
> > 
> > 
> > Thanks
> > Neeraj
> > 
> > > Thanks
> > > 
> > > > ---
> > > >   .../admin-guide/kernel-parameters.txt         | 18 +++++
> > > >   kernel/rcu/srcutree.c                         | 79
> > > > ++++++++++++++-----
> > > >   2 files changed, 78 insertions(+), 19 deletions(-)
> > > > 
> > > > diff --git a/Documentation/admin-guide/kernel-parameters.txt
> > > > b/Documentation/admin-guide/kernel-parameters.txt
> > > > index af647714c113..7e34086c64f5 100644
> > > > --- a/Documentation/admin-guide/kernel-parameters.txt
> > > > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > > > @@ -5805,6 +5805,24 @@
> > > >               expediting.  Set to zero to disable automatic
> > > >               expediting.
> > > > +    srcutree.srcu_max_nodelay [KNL]
> > > > +            Specifies the number of no-delay instances
> > > > +            per jiffy for which the SRCU grace period
> > > > +            worker thread will be rescheduled with zero
> > > > +            delay. Beyond this limit, worker thread will
> > > > +            be rescheduled with a sleep delay of one jiffy.
> > > > +
> > > > +    srcutree.srcu_max_nodelay_phase [KNL]
> > > > +            Specifies the per-grace-period phase, number of
> > > > +            non-sleeping polls of readers. Beyond this limit,
> > > > +            grace period worker thread will be rescheduled
> > > > +            with a sleep delay of one jiffy, between each
> > > > +            rescan of the readers, for a grace period phase.
> > > > +
> > > > +    srcutree.srcu_retry_check_delay [KNL]
> > > > +            Specifies number of microseconds of non-sleeping
> > > > +            delay between each non-sleeping poll of readers.
> > > > +
> > > >       srcutree.small_contention_lim [KNL]
> > > >               Specifies the number of update-side contention
> > > >               events per jiffy will be tolerated before
> > > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > > > index 0db7873f4e95..006828b9c41a 100644
> > > > --- a/kernel/rcu/srcutree.c
> > > > +++ b/kernel/rcu/srcutree.c
> > > > @@ -511,10 +511,49 @@ static bool srcu_readers_active(struct
> > > > srcu_struct *ssp)
> > > >       return sum;
> > > >   }
> > > > -#define SRCU_INTERVAL        1    // Base delay if no expedited
> > > > GPs pending.
> > > > -#define SRCU_MAX_INTERVAL    10    // Maximum incremental delay
> > > > from slow readers.
> > > > -#define SRCU_MAX_NODELAY_PHASE    3    // Maximum per-GP-phase
> > > > consecutive no-delay instances.
> > > > -#define SRCU_MAX_NODELAY    100    // Maximum consecutive
> > > > no-delay instances.
> > > > +/*
> > > > + * We use an adaptive strategy for synchronize_srcu() and
> > > > especially for
> > > > + * synchronize_srcu_expedited().  We spin for a fixed time period
> > > > + * (defined below, boot time configurable) to allow SRCU
> > > > readers to exit
> > > > + * their read-side critical sections.  If there are still some readers
> > > > + * after one jiffy, we repeatedly block for one jiffy time periods.
> > > > + * The blocking time is increased as the grace-period age increases,
> > > > + * with max blocking time capped at 10 jiffies.
> > > > + */
> > > > +#define SRCU_DEFAULT_RETRY_CHECK_DELAY        5
> > > > +
> > > > +static ulong srcu_retry_check_delay = SRCU_DEFAULT_RETRY_CHECK_DELAY;
> > > > +module_param(srcu_retry_check_delay, ulong, 0444);
> > > > +
> > > > +#define SRCU_INTERVAL        1        // Base delay if no
> > > > expedited GPs pending.
> > > > +#define SRCU_MAX_INTERVAL    10        // Maximum incremental
> > > > delay from slow readers.
> > > > +
> > > > +#define SRCU_DEFAULT_MAX_NODELAY_PHASE_LO    3UL    // Lowmark
> > > > on default per-GP-phase
> > > > +                            // no-delay instances.
> > > > +#define SRCU_DEFAULT_MAX_NODELAY_PHASE_HI    1000UL    //
> > > > Highmark on default per-GP-phase
> > > > +                            // no-delay instances.
> > > > +
> > > > +#define SRCU_UL_CLAMP_LO(val, low)    ((val) > (low) ? (val) : (low))
> > > > +#define SRCU_UL_CLAMP_HI(val, high)    ((val) < (high) ? (val)
> > > > : (high))
> > > > +// per-GP-phase no-delay instances adjusted to allow
> > > > non-sleeping poll upto
> > > > +// one jiffies time duration. Mult by 2 is done to factor in
> > > > the srcu_get_delay()
> > > > +// called from process_srcu().
> > > > +#define SRCU_DEFAULT_MAX_NODELAY_PHASE_ADJUSTED    \
> > > > +    (2UL * USEC_PER_SEC / HZ / SRCU_DEFAULT_RETRY_CHECK_DELAY)
> > > > +
> > > > +// Maximum per-GP-phase consecutive no-delay instances.
> > > > +#define SRCU_DEFAULT_MAX_NODELAY_PHASE    (    \
> > > > + SRCU_UL_CLAMP_HI(SRCU_UL_CLAMP_LO(SRCU_DEFAULT_MAX_NODELAY_PHASE_ADJUSTED,
> > > > \
> > > > +                      SRCU_DEFAULT_MAX_NODELAY_PHASE_LO), \
> > > > +             SRCU_DEFAULT_MAX_NODELAY_PHASE_HI))
> > > > +
> > > > +static ulong srcu_max_nodelay_phase = SRCU_DEFAULT_MAX_NODELAY_PHASE;
> > > > +module_param(srcu_max_nodelay_phase, ulong, 0444);
> > > > +
> > > > +#define SRCU_DEFAULT_MAX_NODELAY    100    // Maximum
> > > > consecutive no-delay instances.
> > > > +
> > > > +static ulong srcu_max_nodelay = SRCU_DEFAULT_MAX_NODELAY;
> > > > +module_param(srcu_max_nodelay, ulong, 0444);
> > > >   /*
> > > >    * Return grace-period delay, zero if there are expedited grace
> > > > @@ -535,7 +574,7 @@ static unsigned long srcu_get_delay(struct
> > > > srcu_struct *ssp)
> > > >               jbase += j - gpstart;
> > > >           if (!jbase) {
> > > >               WRITE_ONCE(ssp->srcu_n_exp_nodelay,
> > > > READ_ONCE(ssp->srcu_n_exp_nodelay) + 1);
> > > > -            if (READ_ONCE(ssp->srcu_n_exp_nodelay) >
> > > > SRCU_MAX_NODELAY_PHASE)
> > > > +            if (READ_ONCE(ssp->srcu_n_exp_nodelay) >
> > > > srcu_max_nodelay_phase)
> > > >                   jbase = 1;
> > > >           }
> > > >       }
> > > > @@ -612,15 +651,6 @@ void __srcu_read_unlock(struct srcu_struct
> > > > *ssp, int idx)
> > > >   }
> > > >   EXPORT_SYMBOL_GPL(__srcu_read_unlock);
> > > > -/*
> > > > - * We use an adaptive strategy for synchronize_srcu() and
> > > > especially for
> > > > - * synchronize_srcu_expedited().  We spin for a fixed time period
> > > > - * (defined below) to allow SRCU readers to exit their
> > > > read-side critical
> > > > - * sections.  If there are still some readers after a few
> > > > microseconds,
> > > > - * we repeatedly block for 1-millisecond time periods.
> > > > - */
> > > > -#define SRCU_RETRY_CHECK_DELAY        5
> > > > -
> > > >   /*
> > > >    * Start an SRCU grace period.
> > > >    */
> > > > @@ -706,7 +736,7 @@ static void srcu_schedule_cbs_snp(struct
> > > > srcu_struct *ssp, struct srcu_node *snp
> > > >    */
> > > >   static void srcu_gp_end(struct srcu_struct *ssp)
> > > >   {
> > > > -    unsigned long cbdelay;
> > > > +    unsigned long cbdelay = 1;
> > > >       bool cbs;
> > > >       bool last_lvl;
> > > >       int cpu;
> > > > @@ -726,7 +756,9 @@ static void srcu_gp_end(struct srcu_struct *ssp)
> > > >       spin_lock_irq_rcu_node(ssp);
> > > >       idx = rcu_seq_state(ssp->srcu_gp_seq);
> > > >       WARN_ON_ONCE(idx != SRCU_STATE_SCAN2);
> > > > -    cbdelay = !!srcu_get_delay(ssp);
> > > > +    if (ULONG_CMP_LT(READ_ONCE(ssp->srcu_gp_seq),
> > > > READ_ONCE(ssp->srcu_gp_seq_needed_exp)))
> > > > +        cbdelay = 0;
> > > > +
> > > >       WRITE_ONCE(ssp->srcu_last_gp_end, ktime_get_mono_fast_ns());
> > > >       rcu_seq_end(&ssp->srcu_gp_seq);
> > > >       gpseq = rcu_seq_current(&ssp->srcu_gp_seq);
> > > > @@ -927,12 +959,16 @@ static void srcu_funnel_gp_start(struct
> > > > srcu_struct *ssp, struct srcu_data *sdp,
> > > >    */
> > > >   static bool try_check_zero(struct srcu_struct *ssp, int idx,
> > > > int trycount)
> > > >   {
> > > > +    unsigned long curdelay;
> > > > +
> > > > +    curdelay = !srcu_get_delay(ssp);
> > > > +
> > > >       for (;;) {
> > > >           if (srcu_readers_active_idx_check(ssp, idx))
> > > >               return true;
> > > > -        if (--trycount + !srcu_get_delay(ssp) <= 0)
> > > > +        if ((--trycount + curdelay) <= 0)
> > > >               return false;
> > > > -        udelay(SRCU_RETRY_CHECK_DELAY);
> > > > +        udelay(srcu_retry_check_delay);
> > > >       }
> > > >   }
> > > > @@ -1588,7 +1624,7 @@ static void process_srcu(struct
> > > > work_struct *work)
> > > >           j = jiffies;
> > > >           if (READ_ONCE(ssp->reschedule_jiffies) == j) {
> > > >               WRITE_ONCE(ssp->reschedule_count,
> > > > READ_ONCE(ssp->reschedule_count) + 1);
> > > > -            if (READ_ONCE(ssp->reschedule_count) > SRCU_MAX_NODELAY)
> > > > +            if (READ_ONCE(ssp->reschedule_count) > srcu_max_nodelay)
> > > >                   curdelay = 1;
> > > >           } else {
> > > >               WRITE_ONCE(ssp->reschedule_count, 1);
> > > > @@ -1680,6 +1716,11 @@ static int __init srcu_bootup_announce(void)
> > > >       pr_info("Hierarchical SRCU implementation.\n");
> > > >       if (exp_holdoff != DEFAULT_SRCU_EXP_HOLDOFF)
> > > >           pr_info("\tNon-default auto-expedite holdoff of %lu
> > > > ns.\n", exp_holdoff);
> > > > +    if (srcu_retry_check_delay != SRCU_DEFAULT_RETRY_CHECK_DELAY)
> > > > +        pr_info("\tNon-default retry check delay of %lu us.\n",
> > > > srcu_retry_check_delay);
> > > > +    if (srcu_max_nodelay != SRCU_DEFAULT_MAX_NODELAY)
> > > > +        pr_info("\tNon-default max no-delay of %lu.\n",
> > > > srcu_max_nodelay);
> > > > +    pr_info("\tMax phase no-delay instances is %lu.\n",
> > > > srcu_max_nodelay_phase);
> > > >       return 0;
> > > >   }
> > > >   early_initcall(srcu_bootup_announce);
> > > 
>