linux-kernel - Re: [PATCH v4 00/10] PCID and improved laziness

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Message-ID: <CALCETrVf87m6CRG3-m=i3wP5DyD5gfcMVJA4KDXb8TarCps2iA@mail.gmail.com>
Date:   Tue, 11 Jul 2017 08:00:47 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Matt Fleming <matt@...eblueprint.co.uk>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Nadav Amit <nadav.amit@...il.com>,
        Rik van Riel <riel@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v4 00/10] PCID and improved laziness

On Tue, Jul 11, 2017 at 4:32 AM, Matt Fleming <matt@...eblueprint.co.uk> wrote:
> On Fri, 30 Jun, at 01:44:22PM, Matt Fleming wrote:
>> On Thu, 29 Jun, at 08:53:12AM, Andy Lutomirski wrote:
>> > *** Ingo, even if this misses 4.13, please apply the first patch before
>> > *** the merge window.
>> >
>> > There are three performance benefits here:
>> >
>> > 1. TLB flushing is slow.  (I.e. the flush itself takes a while.)
>> >    This avoids many of them when switching tasks by using PCID.  In
>> >    a stupid little benchmark I did, it saves about 100ns on my laptop
>> >    per context switch.  I'll try to improve that benchmark.
>> >
>> > 2. Mms that have been used recently on a given CPU might get to keep
>> >    their TLB entries alive across process switches with this patch
>> >    set.  TLB fills are pretty fast on modern CPUs, but they're even
>> >    faster when they don't happen.
>> >
>> > 3. Lazy TLB is way better.  We used to do two stupid things when we
>> >    ran kernel threads: we'd send IPIs to flush user contexts on their
>> >    CPUs and then we'd write to CR3 for no particular reason as an excuse
>> >    to stop further IPIs.  With this patch, we do neither.
>>
>> Heads up, I'm gonna queue this for a run on SUSE's performance test
>> grid.
>
> FWIW, I didn't see any change in performance with this series on a
> PCID-capable machine. On the plus side, I didn't see any weird-looking
> bugs either.
>
> Are your benchmarks available anywhere?

https://git.kernel.org/pub/scm/linux/kernel/git/luto/misc-tests.git/

I did:

$ ./context_switch_latency_64 0 process same

and

$ ./madvise_bounce_64 10k [IIRC -- it might have been a different loop count]

--Andy