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

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Message-ID: <ZD6yirD6Ob+1xG32@casper.infradead.org>
Date:   Tue, 18 Apr 2023 16:08:58 +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 Tue, Apr 18, 2023 at 07:54:01AM -0700, Suren Baghdasaryan wrote:
> On Tue, Apr 18, 2023 at 7:25 AM Matthew Wilcox <willy@...radead.org> wrote:
> >
> > 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.
> 
> 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?

I think so.  Let's see your patch and then we can be sure we're talking
about the same thing ;-)