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

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Message-ID: <20170630124422.GA12077@codeblueprint.co.uk>
Date:   Fri, 30 Jun 2017 13:44:22 +0100
From:   Matt Fleming <matt@...eblueprint.co.uk>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     x86@...nel.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 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.