linux-kernel - Re: [PATCH 2/3] mm/cow: optimise pte dirty/accessed bits handling in fork

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Message-ID: <CA+55aFzBHNsLNs4TfOrMQXTsV9u8=7yAu4GbsOM84AQb-OhJmg@mail.gmail.com>
Date:   Wed, 29 Aug 2018 16:15:37 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Nick Piggin <npiggin@...il.com>
Cc:     linux-mm <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/3] mm/cow: optimise pte dirty/accessed bits handling in fork

On Wed, Aug 29, 2018 at 4:12 PM Nicholas Piggin <npiggin@...il.com> wrote:
>
> Dirty micro fault seems to be the big one for my Skylake, takes 300
> nanoseconds per access. Accessed takes about 100. (I think, have to
> go over my benchmark a bit more carefully and re-test).

Yeah, but they only happen for shared areas after fork, which sounds
like it shouldn't be a big deal in most cases.

And I'm not entirely objecting to your patch per se, I just would want
to keep the accessed bit changes separate from the dirty bit ones.

*If* somebody has bisectable issues with it (performance or not), it
will then be clearer what the exact issue is.

            Linus