linux-kernel - Re: [PATCH 1/8] perf/hw_breakpoint: Optimize list of per-task breakpoints

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Date:   Thu, 9 Jun 2022 15:05:40 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Marco Elver <elver@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-perf-users@...r.kernel.org, x86@...nel.org,
        linux-sh@...r.kernel.org, kasan-dev@...glegroups.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/8] perf/hw_breakpoint: Optimize list of per-task breakpoints

On Thu, 9 Jun 2022 at 14:53, Marco Elver <elver@...gle.com> wrote:
>
> On Thu, Jun 09, 2022 at 02:30PM +0200, Dmitry Vyukov wrote:
> [...]
> > > +       rcu_read_lock();
> >
> > Why do we need rcu_read_lock() here?
> > The patch does not change anything with respect to locking, so all
> > accesses to the container should still be protected by nr_bp_mutex.
> > Similarly for the rcu variant of for_each below.
> [...]
> > > +       head = rhltable_lookup(&task_bps_ht, &bp->hw.target, task_bps_ht_params);
> > > +       if (!head)
> > > +               goto out;
> > > +
> > > +       rhl_for_each_entry_rcu(iter, pos, head, hw.bp_list) {
>
> It's part of rhashtable's interface requirements:
>
>         /**
>          * rhltable_lookup - search hash list table
>          * @hlt:        hash table
>          * @key:        the pointer to the key
>          * @params:     hash table parameters
>          *
>          * Computes the hash value for the key and traverses the bucket chain looking
>          * for a entry with an identical key.  All matching entries are returned
>          * in a list.
>          *
>          * This must only be called under the RCU read lock.
>          *
>          * Returns the list of entries that match the given key.
>          */
>
> Beyond that, even though there might not appear to be any concurrent
> rhashtable modifications, it'll be allowed in patch 6/8. Furthermore,
> rhashtable actually does concurrent background compactions since I
> selected 'automatic_shrinking = true' (so we don't leak tons of memory
> after starting and killing those 1000s of tasks) -- there's this
> call_rcu() in lib/rhashtable.c that looks like that's when it's used.
> This work is done in a deferred work by rht_deferred_worker().


I see. Thanks.

Reviewed-by: Dmitry Vyukov <dvyukov@...gle.com>