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

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Date:   Fri, 9 Sep 2016 09:44:56 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...capital.net>
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 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.