linux-kernel - Re: [PATCH RFC v6] x86,mm,sched: make lazy TLB mode even lazier

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Message-ID: <CALCETrVGYH4xpB06Nchj7HSZUBOFGgGFbJJBvpX52xD=vkaJnw@mail.gmail.com>
Date:   Mon, 24 Apr 2017 20:30:19 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Benjamin Serebrin <serebrin@...gle.com>,
        Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...e.de>, Mel Gorman <mgorman@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH RFC v6] x86,mm,sched: make lazy TLB mode even lazier

On Fri, Sep 9, 2016 at 12:44 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, Sep 08, 2016 at 09:39:45PM -0700, Andy Lutomirski wrote:
>> If they're busy threads, shouldn't the yield return immediately
>> because the threads are still ready to run?  Lazy TLB won't do much
>> unless you get the kernel in some state where it's running in the
>> context of a different kernel thread and hasn't switched to
>> swapper_pg_dir.  IIRC idle works like that, but you'd need to actually
>> sleep to go idle.
>
> Right, a task doing:
>
>         for (;;) sched_yield();
>
> esp. when its the only runnable thread on the CPU, is a busy thread. It
> will not enter switch_mm(), which was where the invalidate hook was
> placed IIRC.

Hi all-

I'm guessing that this patch got abandoned, at least temporarily.  I'm
currently polishing up my PCID series, and I think it might be worth
revisiting this on top of my PCID rework.  The relevant major
infrastructure change I'm making with my PCID code is that I'm adding
an atomic64_t to each mm_context_t that gets incremented every time a
flush on that mm is requested.  With that change, we might be able to
get away with simply removing a cpu from mm_cpumask immediately when
it enters lazy mode and adding a hook to the scheduler to revalidate
the TLB state when switching mms when we were previously lazy.
Revalidation would just check that the counter hasn't changed.

--Andy