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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Tue, 18 Apr 2023 07:54:01 -0700
Message-ID: <CAJuCfpFJ0owZELS2COukb0rHCOpqNMW-x9vVonkhknReZb=Zsg@mail.gmail.com>
Subject: Re: [PATCH v2 1/1] mm: do not increment pgfault stats when page fault
 handler retries
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Peter Xu <peterx@redhat.com>, akpm@linux-foundation.org,
        hannes@cmpxchg.org, mhocko@suse.com, josef@toxicpanda.com,
        jack@suse.cz, ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com,
        michel@lespinasse.org, liam.howlett@oracle.com, jglisse@google.com,
        vbabka@suse.cz, minchan@google.com, dave@stgolabs.net,
        punit.agrawal@bytedance.com, lstoakes@gmail.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        kernel-team@android.com
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On Tue, Apr 18, 2023 at 7:25=E2=80=AFAM Matthew Wilcox <willy@infradead.org=
> wrote:
>
> On Mon, Apr 17, 2023 at 04:17:45PM -0700, Suren Baghdasaryan wrote:
> > On Mon, Apr 17, 2023 at 3:52=E2=80=AFPM Matthew Wilcox <willy@infradead=
.org> wrote:
> > >
> > > On Mon, Apr 17, 2023 at 03:40:33PM -0400, Peter Xu wrote:
> > > > >     /*
> > > > > -    * We don't do accounting for some specific faults:
> > > > > -    *
> > > > > -    * - Unsuccessful faults (e.g. when the address wasn't valid)=
.  That
> > > > > -    *   includes arch_vma_access_permitted() failing before reac=
hing here.
> > > > > -    *   So this is not a "this many hardware page faults" counte=
r.  We
> > > > > -    *   should use the hw profiling for that.
> > > > > -    *
> > > > > -    * - Incomplete faults (VM_FAULT_RETRY).  They will only be c=
ounted
> > > > > -    *   once they're completed.
> > > > > +    * Do not account for incomplete faults (VM_FAULT_RETRY). The=
y will be
> > > > > +    * counted upon completion.
> > > > >      */
> > > > > -   if (ret & (VM_FAULT_ERROR | VM_FAULT_RETRY))
> > > > > +   if (ret & VM_FAULT_RETRY)
> > > > > +           return;
> > > > > +
> > > > > +   /* Register both successful and failed faults in PGFAULT coun=
ters. */
> > > > > +   count_vm_event(PGFAULT);
> > > > > +   count_memcg_event_mm(mm, PGFAULT);
> > > >
> > > > Is there reason on why vm events accountings need to be explicitly
> > > > different from perf events right below on handling ERROR?
> > >
> > > I think so.  ERROR is quite different from RETRY.  If we are, for
> > > example, handling a SIGSEGV (perhaps a GC language?) that should be
> > > accounted.  If we can't handle a page fault right now, and need to
> > > retry within the kernel, that should not be accounted.
> >
> > IIUC, the question was about the differences in vm vs perf accounting
> > for errors, not the difference between ERROR and RETRY cases. Matthew,
> > are you answering the right question or did I misunderstand your
> > answer?
>
> Maybe I'm misunderstanding what you're proposing.  I thought the
> proposal was to make neither ERROR nor RETRY increment the counters,
> but if the proposal is to make ERROR increment the perf counters
> instead, then that's cool with me.

Oh, I think now I understand your answer. You were not highlighting
the difference between the who but objecting to the proposal of not
counting both ERROR and RETRY. Am I on the same page now?