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Date: Thu, 3 Sep 2020 16:32:54 +0800 From: Alex Shi <alex.shi@...ux.alibaba.com> To: Mel Gorman <mgorman@...hsingularity.net> Cc: Anshuman Khandual <anshuman.khandual@....com>, David Hildenbrand <david@...hat.com>, Matthew Wilcox <willy@...radead.org>, Vlastimil Babka <vbabka@...e.cz>, Alexander Duyck <alexander.duyck@...il.com>, Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH v4 1/4] mm/pageblock: mitigation cmpxchg false sharing in pageblock flags 在 2020/9/3 下午3:24, Mel Gorman 写道: > On Thu, Sep 03, 2020 at 03:01:20PM +0800, Alex Shi wrote: >> pageblock_flags is used as long, since every pageblock_flags is just 4 >> bits, 'long' size will include 8(32bit machine) or 16 pageblocks' flags, >> that flag setting has to sync in cmpxchg with 7 or 15 other pageblock >> flags. It would cause long waiting for sync. >> >> If we could change the pageblock_flags variable as char, we could use >> char size cmpxchg, which just sync up with 2 pageblock flags. it could >> relief the false sharing in cmpxchg. >> >> Signed-off-by: Alex Shi <alex.shi@...ux.alibaba.com> > > Page block types were not known to change at high frequency that would > cause a measurable performance drop. If anything, the performance hit > from pageblocks is the lookup paths which is a lot more frequent. Yes, it is not hot path. But it's still a meaningful points to reduce cmpxchg level false sharing which isn't right on logical. > > What was the workload you were running that altered pageblocks at a high > enough frequency for collisions to occur when updating adjacent > pageblocks? > I have run thpscale with 'always' defrag setting of THP. The Amean stddev is much larger than a very little average run time reducing. But the left patch 4 could show the cmpxchg retry reduce from thousands to hundreds or less. Subject: [PATCH v4 4/4] add cmpxchg tracing Signed-off-by: Alex Shi <alex.shi@...ux.alibaba.com> --- include/trace/events/pageblock.h | 30 ++++++++++++++++++++++++++++++ mm/page_alloc.c | 4 ++++ 2 files changed, 34 insertions(+) create mode 100644 include/trace/events/pageblock.h diff --git a/include/trace/events/pageblock.h b/include/trace/events/pageblock.h new file mode 100644 index 000000000000..003c2d716f82 --- /dev/null +++ b/include/trace/events/pageblock.h @@ -0,0 +1,30 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#undef TRACE_SYSTEM +#define TRACE_SYSTEM pageblock + +#if !defined(_TRACE_PAGEBLOCK_H) || defined(TRACE_HEADER_MULTI_READ) +#define _TRACE_PAGEBLOCK_H + +#include <linux/tracepoint.h> + +TRACE_EVENT(hit_cmpxchg, + + TP_PROTO(char byte), + + TP_ARGS(byte), + + TP_STRUCT__entry( + __field(char, byte) + ), + + TP_fast_assign( + __entry->byte = byte; + ), + + TP_printk("%d", __entry->byte) +); + +#endif /* _TRACE_PAGE_ISOLATION_H */ + +/* This part must be outside protection */ +#include <trace/define_trace.h> diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 8b65d83d8be6..a6d7159295bc 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -509,6 +509,9 @@ static __always_inline int get_pfnblock_migratetype(struct page *page, unsigned * @pfn: The target page frame number * @mask: mask of bits that the caller is interested in */ +#define CREATE_TRACE_POINTS +#include <trace/events/pageblock.h> + void set_pfnblock_flags_mask(struct page *page, unsigned long flags, unsigned long pfn, unsigned long mask) @@ -532,6 +535,7 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags, if (byte == old_byte) break; byte = old_byte; + trace_hit_cmpxchg(byte); } } -- 1.8.3.1
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