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

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 :/

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