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From:   SeongJae Park <sjpark@amazon.com>
To:     Eric Dumazet <edumazet@google.com>
CC:     SeongJae Park <sjpark@amazon.com>,
        David Miller <davem@davemloft.net>,
        SeongJae Park <sjpark@amazon.de>,
        Jakub Kicinski <kuba@kernel.org>,
        "Alexey Kuznetsov" <kuznet@ms2.inr.ac.ru>,
        Florian Westphal <fw@strlen.de>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        netdev <netdev@vger.kernel.org>, <rcu@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v3 1/1] net/ipv4/inet_fragment: Batch fqdir destroy works
Date:   Fri, 11 Dec 2020 11:46:22 +0100
Message-ID: <20201211104622.23231-1-sjpark@amazon.com>
In-Reply-To: <CANn89i+P8d8Ok8k1o3_ADW4iWLKU=qikq+RAxmqkYbUn1wkWvQ@mail.gmail.com>
MIME-Version: 1.0
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On Fri, 11 Dec 2020 11:41:36 +0100 Eric Dumazet <edumazet@google.com> wrote:

> On Fri, Dec 11, 2020 at 11:33 AM SeongJae Park <sjpark@amazon.com> wrote:
> >
> > On Fri, 11 Dec 2020 09:43:41 +0100 Eric Dumazet <edumazet@google.com> wrote:
> >
> > > On Fri, Dec 11, 2020 at 9:21 AM SeongJae Park <sjpark@amazon.com> wrote:
> > > >
> > > > From: SeongJae Park <sjpark@amazon.de>
> > > >
> > > > For each 'fqdir_exit()' call, a work for destroy of the 'fqdir' is
> > > > enqueued.  The work function, 'fqdir_work_fn()', internally calls
> > > > 'rcu_barrier()'.  In case of intensive 'fqdir_exit()' (e.g., frequent
> > > > 'unshare()' systemcalls), this increased contention could result in
> > > > unacceptably high latency of 'rcu_barrier()'.  This commit avoids such
> > > > contention by doing the 'rcu_barrier()' and subsequent lightweight works
> > > > in a batched manner using a dedicated singlethread worker, as similar to
> > > > that of 'cleanup_net()'.
> > >
> > >
> > > Not sure why you submit a patch series with a single patch.
> > >
> > > Your cover letter contains interesting info that would better be
> > > captured in this changelog IMO
> >
> > I thought someone might think this is not a kernel issue but the reproducer is
> > insane or 'rcu_barrier()' needs modification.  I wanted to do such discussion
> > on the coverletter.  Seems I misjudged.  I will make this single patch and move
> > the detailed information here from the next version.
> >
> > >
> > > >
> > > > Signed-off-by: SeongJae Park <sjpark@amazon.de>
> > > > ---
> > > >  include/net/inet_frag.h  |  1 +
> > > >  net/ipv4/inet_fragment.c | 45 +++++++++++++++++++++++++++++++++-------
> > > >  2 files changed, 39 insertions(+), 7 deletions(-)
> > > >
> > > > diff --git a/include/net/inet_frag.h b/include/net/inet_frag.h
> > > > index bac79e817776..48cc5795ceda 100644
> > > > --- a/include/net/inet_frag.h
> > > > +++ b/include/net/inet_frag.h
> > > > @@ -21,6 +21,7 @@ struct fqdir {
> > > >         /* Keep atomic mem on separate cachelines in structs that include it */
> > > >         atomic_long_t           mem ____cacheline_aligned_in_smp;
> > > >         struct work_struct      destroy_work;
> > > > +       struct llist_node       free_list;
> > > >  };
> > > >
> > > >  /**
> > > > diff --git a/net/ipv4/inet_fragment.c b/net/ipv4/inet_fragment.c
> > > > index 10d31733297d..a6fc4afcc323 100644
> > > > --- a/net/ipv4/inet_fragment.c
> > > > +++ b/net/ipv4/inet_fragment.c
> > > > @@ -145,12 +145,17 @@ static void inet_frags_free_cb(void *ptr, void *arg)
> > > >                 inet_frag_destroy(fq);
> > > >  }
> > > >
> > > > -static void fqdir_work_fn(struct work_struct *work)
> > > > +static struct workqueue_struct *fqdir_wq;
> > > > +static LLIST_HEAD(free_list);
> > > > +
> > > > +static void fqdir_free_fn(struct work_struct *work)
> > > >  {
> > > > -       struct fqdir *fqdir = container_of(work, struct fqdir, destroy_work);
> > > > -       struct inet_frags *f = fqdir->f;
> > > > +       struct llist_node *kill_list;
> > > > +       struct fqdir *fqdir, *tmp;
> > > > +       struct inet_frags *f;
> > > >
> > > > -       rhashtable_free_and_destroy(&fqdir->rhashtable, inet_frags_free_cb, NULL);
> > > > +       /* Atomically snapshot the list of fqdirs to free */
> > > > +       kill_list = llist_del_all(&free_list);
> > > >
> > > >         /* We need to make sure all ongoing call_rcu(..., inet_frag_destroy_rcu)
> > > >          * have completed, since they need to dereference fqdir.
> > > > @@ -158,12 +163,38 @@ static void fqdir_work_fn(struct work_struct *work)
> > > >          */
> > > >         rcu_barrier();
> > > >
> > > > -       if (refcount_dec_and_test(&f->refcnt))
> > > > -               complete(&f->completion);
> > > > +       llist_for_each_entry_safe(fqdir, tmp, kill_list, free_list) {
> > > > +               f = fqdir->f;
> > > > +               if (refcount_dec_and_test(&f->refcnt))
> > > > +                       complete(&f->completion);
> > > >
> > > > -       kfree(fqdir);
> > > > +               kfree(fqdir);
> > > > +       }
> > > >  }
> > > >
> > > > +static DECLARE_WORK(fqdir_free_work, fqdir_free_fn);
> > > > +
> > > > +static void fqdir_work_fn(struct work_struct *work)
> > > > +{
> > > > +       struct fqdir *fqdir = container_of(work, struct fqdir, destroy_work);
> > > > +
> > > > +       rhashtable_free_and_destroy(&fqdir->rhashtable, inet_frags_free_cb, NULL);
> > > > +
> > > > +       if (llist_add(&fqdir->free_list, &free_list))
> > > > +               queue_work(fqdir_wq, &fqdir_free_work);
> > >
> > > I think you misunderstood me.
> > >
> > > Since this fqdir_free_work will have at most one instance, you can use
> > > system_wq here, there is no risk of abuse.
> > >
> > > My suggestion was to not use system_wq for fqdir_exit(), to better
> > > control the number
> > >  of threads in rhashtable cleanups.
> > >
> > > void fqdir_exit(struct fqdir *fqdir)
> > > {
> > >         INIT_WORK(&fqdir->destroy_work, fqdir_work_fn);
> > > -       queue_work(system_wq, &fqdir->destroy_work);
> > > +      queue_work(fqdir_wq, &fqdir->destroy_work);
> > > }
> >
> > Oh, got it.  I definitely misunderstood.  My fault, sorry.
> >
> > >
> > >
> > >
> > > > +}
> > > > +
> > > > +static int __init fqdir_wq_init(void)
> > > > +{
> > > > +       fqdir_wq = create_singlethread_workqueue("fqdir");
> > >
> > >
> > > And here, I suggest to use a non ordered work queue, allowing one
> > > thread per cpu, to allow concurrent rhashtable cleanups
> > >
> > > Also "fqdir" name is rather vague, this is an implementation detail ?
> > >
> > > fqdir_wq =create_workqueue("inet_frag_wq");
> >
> > So, what you are suggesting is to use a dedicated non-ordered work queue
> > (fqdir_wq) for rhashtable cleanup and do the remaining works with system_wq in
> > the batched manner, right?  IOW, doing below change on top of this patch.
> >
> > --- a/net/ipv4/inet_fragment.c
> > +++ b/net/ipv4/inet_fragment.c
> > @@ -145,7 +145,7 @@ static void inet_frags_free_cb(void *ptr, void *arg)
> >                 inet_frag_destroy(fq);
> >  }
> >
> > -static struct workqueue_struct *fqdir_wq;
> > +static struct workqueue_struct *inet_frag_wq;
> >  static LLIST_HEAD(free_list);
> 
> Nit : Please prefix this free_list , like fqdir_free_list  to avoid
> namespace pollution.

Sure!

> 
> 
> >
> >  static void fqdir_free_fn(struct work_struct *work)
> > @@ -181,14 +181,14 @@ static void fqdir_work_fn(struct work_struct *work)
> >         rhashtable_free_and_destroy(&fqdir->rhashtable, inet_frags_free_cb, NULL);
> >
> >         if (llist_add(&fqdir->free_list, &free_list))
> > -               queue_work(fqdir_wq, &fqdir_free_work);
> > +               queue_work(system_wq, &fqdir_free_work);
> >  }
> >
> >  static int __init fqdir_wq_init(void)
> >  {
> > -       fqdir_wq = create_singlethread_workqueue("fqdir");
> > -       if (!fqdir_wq)
> > -               panic("Could not create fqdir workq");
> > +       inet_frag_wq = create_workqueue("inet_frag_wq");
> > +       if (!inet_frag_wq)
> > +               panic("Could not create inet frag workq");
> >         return 0;
> >  }
> >
> > @@ -218,7 +218,7 @@ EXPORT_SYMBOL(fqdir_init);
> >  void fqdir_exit(struct fqdir *fqdir)
> >  {
> >         INIT_WORK(&fqdir->destroy_work, fqdir_work_fn);
> > -       queue_work(system_wq, &fqdir->destroy_work);
> > +       queue_work(inet_frag_wq, &fqdir->destroy_work);
> >  }
> >  EXPORT_SYMBOL(fqdir_exit);
> >
> > If I'm still misunderstanding, please let me know.
> >
> 
> I think that with the above changes, we should be good ;)

Thank you for your quick and nice reply.  I will send the next version soon!


Thanks,
SeongJae Park