linux-kernel - Re: [PATCH 4/4] mm: migrate: Batch TLB flushing when unmapping pages for migration

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Date:	Thu, 16 Apr 2015 11:51:57 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Linux-MM <linux-mm@...ck.org>
Cc:	Rik van Riel <riel@...hat.com>, Hugh Dickins <hughd@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	Dave Hansen <dave.hansen@...el.com>,
	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] mm: migrate: Batch TLB flushing when unmapping pages
 for migration

On Thu, Apr 16, 2015 at 11:22:46AM +0100, Mel Gorman wrote:
> Page reclaim batches multiple TLB flushes into one IPI and this patch teaches
> page migration to also batch any necessary flushes. MMtests has a THP scale
> microbenchmark that deliberately fragments memory and then allocates THPs
> to stress compaction. It's not a page reclaim benchmark and recent kernels
> avoid excessive compaction but this patch reduced system CPU usage
> 
>                4.0.0       4.0.0
>             baseline batchmigrate-v1
> User          970.70     1012.24
> System       2067.48     1840.00
> Elapsed      1520.63     1529.66
> 
> Note that this particular workload was not TLB flush intensive with peaks
> in interrupts during the compaction phase. The 4.0 kernel peaked at 345K
> interrupts/second, the kernel that batches reclaim TLB entries peaked at
> 13K interrupts/second and this patch peaked at 10K interrupts/second.
> 
> Signed-off-by: Mel Gorman <mgorman@...e.de>

The following is needed on !x86 although it's pointless to test on !x86
at the moment.

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 361bf59e0594..548c94834112 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2784,10 +2784,6 @@ void alloc_tlb_ubc(void)
 	cpumask_clear(&current->tlb_ubc->cpumask);
 	current->tlb_ubc->nr_pages = 0;
 }
-#else
-static inline void alloc_tlb_ubc(void)
-{
-}
 #endif /* CONFIG_ARCH_SUPPORTS_LOCAL_TLB_PFN_FLUSH */
 
 unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
--
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