RE: [PATCH 1/2] sched/wait: Break up long wake list walk

From: "Liang, Kan" <kan.liang@intel.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: 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>,
	Peter Zijlstra <peterz@infradead.org>,
	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>
Subject: RE: [PATCH 1/2] sched/wait: Break up long wake list walk
Date: Wed, 23 Aug 2017 14:49:31 +0000	[thread overview]
Message-ID: <37D7C6CF3E00A74B8858931C1DB2F0775378A8AB@SHSMSX103.ccr.corp.intel.com> (raw)
In-Reply-To: <CA+55aFwavpFfKNW9NVgNhLggqhii-guc5aX1X5fxrPK+==id0g@mail.gmail.com>

> On Tue, Aug 22, 2017 at 12:55 PM, Liang, Kan <kan.liang@intel.com> wrote:
> >
> >> So I propose testing the attached trivial patch.
> >
> > It doesn’t work.
> > The call stack is the same.
> 
> So I would have expected the stack trace to be the same, and I would even
> expect the CPU usage to be fairly similar, because you'd see repeating from
> the callers (taking the fault again if the page is - once again - being migrated).
> 
> But I was hoping that the wait queues would be shorter because the loop for
> the retry would be bigger.
> 
> Oh well.
> 
> I'm slightly out of ideas. Apparently the yield() worked ok (apart from not
> catching all cases), and maybe we could do a version that waits on the page
> bit in the non-contended case, but yields under contention?
> 
> IOW, maybe this is the best we can do for now? Introducing that
> "wait_on_page_migration()" helper might allow us to tweak this a bit as
> people come up with better ideas..

The "wait_on_page_migration()" helper works well in the overnight testing.

Thanks,
Kan

> 
> And then add Tim's patch for the general worst-case just in case?
> 
>              Linus