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Message-ID: <1500022977.2865.88.camel@kernel.crashing.org>
Subject: Re: Potential race in TLB flush batching?
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Date: Fri, 14 Jul 2017 19:02:57 +1000
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To: Mel Gorman <mgorman@suse.de>
Cc: Andy Lutomirski <luto@kernel.org>, Nadav Amit <nadav.amit@gmail.com>, linux-mm@kvack.org, "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>

On Fri, 2017-07-14 at 09:31 +0100, Mel Gorman wrote:
> It may also be only a gain on a limited number of architectures depending
> on exactly how an architecture handles flushing. At the time, batching
> this for x86 in the worse-case scenario where all pages being reclaimed
> were mapped from multiple threads knocked 24.4% off elapsed run time and
> 29% off system CPU but only on multi-socket NUMA machines. On UMA, it was
> barely noticable. For some workloads where only a few pages are mapped or
> the mapped pages on the LRU are relatively sparese, it'll make no difference.
> 
> The worst-case situation is extremely IPI intensive on x86 where many
> IPIs were being sent for each unmap. It's only worth even considering if
> you see that the time spent sending IPIs for flushes is a large portion
> of reclaim.

Ok, it would be interesting to see how that compares to powerpc with
its HW tlb invalidation broadcasts. We tend to hate them and prefer
IPIs in most cases but maybe not *this* case .. (mostly we find that
IPI + local inval is better for large scale invals, such as full mm on
exit/fork etc...).

In the meantime I found the original commits, we'll dig and see if it's
useful for us.

Cheers,
Ben.

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