linux-kernel - Re: [PATCH 1/2] sched/wait: Break up long wake list walk

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Message-ID: <20170822205316.GS32112@worktop.programming.kicks-ass.net>
Date:   Tue, 22 Aug 2017 22:53:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Liang, Kan" <kan.liang@...el.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mel Gorman <mgorman@...e.de>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Ingo Molnar <mingo@...e.hu>, Andi Kleen <ak@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>, Jan Kara <jack@...e.cz>,
        linux-mm <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] sched/wait: Break up long wake list walk

On Tue, Aug 22, 2017 at 01:42:13PM -0700, Linus Torvalds wrote:
> +void wait_on_page_bit_or_yield(struct page *page, int bit_nr)
> +{
> +	if (PageWaiters(page)) {
> +		yield();
> +		return;
> +	}
> +	wait_on_page_bit(page, bit_nr);
> +}

So _the_ problem with yield() is when you hit this with a RT task it
will busy spin and possibly not allow the task that actually has the
lock to make progress at all.

So ideally there'd be a timeout or other limit on the amount of yield().

This being bit-spinlocks leaves us very short on state to play with
though :/