Re: [PATCH v3 1/1] net/ipv4/inet_fragment: Batch fqdir destroy works

[Eric Dumazet]

On Fri, Dec 11, 2020 at 11:33 AM SeongJae Park <sjpark@xxxxxxxxxx> wrote:
>
> On Fri, 11 Dec 2020 09:43:41 +0100 Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>
> > On Fri, Dec 11, 2020 at 9:21 AM SeongJae Park <sjpark@xxxxxxxxxx> wrote:
> > >
> > > From: SeongJae Park <sjpark@xxxxxxxxx>
> > >
> > > 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@xxxxxxxxx>
> > > ---
> > >  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.


>
>  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 ;)

Thanks !