linux-kernel - Re: [PATCH v2 1/1] mm: do not increment pgfault stats when page fault handler retries

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Date:   Tue, 18 Apr 2023 15:25:26 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Peter Xu <peterx@...hat.com>, akpm@...ux-foundation.org,
        hannes@...xchg.org, mhocko@...e.com, josef@...icpanda.com,
        jack@...e.cz, ldufour@...ux.ibm.com, laurent.dufour@...ibm.com,
        michel@...pinasse.org, liam.howlett@...cle.com, jglisse@...gle.com,
        vbabka@...e.cz, minchan@...gle.com, dave@...olabs.net,
        punit.agrawal@...edance.com, lstoakes@...il.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v2 1/1] mm: do not increment pgfault stats when page
 fault handler retries

On Mon, Apr 17, 2023 at 04:17:45PM -0700, Suren Baghdasaryan wrote:
> On Mon, Apr 17, 2023 at 3:52 PM Matthew Wilcox <willy@...radead.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 reaching here.
> > > > -    *   So this is not a "this many hardware page faults" counter.  We
> > > > -    *   should use the hw profiling for that.
> > > > -    *
> > > > -    * - Incomplete faults (VM_FAULT_RETRY).  They will only be counted
> > > > -    *   once they're completed.
> > > > +    * Do not account for incomplete faults (VM_FAULT_RETRY). They 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 counters. */
> > > > +   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.