[patch 016/262] mm, slub: use prefetchw instead of prefetch

Andrew Morton
2021-11-05T20:35:27Z

From: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Subject: mm, slub: use prefetchw instead of prefetch

commit 0ad9500e16fe ("slub: prefetch next freelist pointer in
slab_alloc()") introduced prefetch_freepointer() because when other cpu(s)
freed objects into a page that current cpu owns, the freelist link is hot
on cpu(s) which freed objects and possibly very cold on current cpu.

But if freelist link chain is hot on cpu(s) which freed objects,
it's better to invalidate that chain because they're not going to access
again within a short time.

So use prefetchw instead of prefetch. On supported architectures like x86
and arm, it invalidates other copied instances of a cache line when
prefetching it.

Before:

Time: 91.677

Performance counter stats for 'hackbench -g 100 -l 10000':
1462938.07 msec cpu-clock # 15.908 CPUs utilized
18072550 context-switches # 12.354 K/sec
1018814 cpu-migrations # 696.416 /sec
104558 page-faults # 71.471 /sec
1580035699271 cycles # 1.080 GHz (54.51%)
2003670016013 instructions # 1.27 insn per cycle (54.31%)
5702204863 branch-misses (54.28%)
643368500985 cache-references # 439.778 M/sec (54.26%)
18475582235 cache-misses # 2.872 % of all cache refs (54.28%)
642206796636 L1-dcache-loads # 438.984 M/sec (46.87%)
18215813147 L1-dcache-load-misses # 2.84% of all L1-dcache accesses (46.83%)
653842996501 dTLB-loads # 446.938 M/sec (46.63%)
3227179675 dTLB-load-misses # 0.49% of all dTLB cache accesses (46.85%)
537531951350 iTLB-loads # 367.433 M/sec (54.33%)
114750630 iTLB-load-misses # 0.02% of all iTLB cache accesses (54.37%)
630135543177 L1-icache-loads # 430.733 M/sec (46.80%)
22923237620 L1-icache-load-misses # 3.64% of all L1-icache accesses (46.76%)

91.964452802 seconds time elapsed

43.416742000 seconds user
1422.441123000 seconds sys

After:

Time: 90.220

Performance counter stats for 'hackbench -g 100 -l 10000':
1437418.48 msec cpu-clock # 15.880 CPUs utilized
17694068 context-switches # 12.310 K/sec
958257 cpu-migrations # 666.651 /sec
100604 page-faults # 69.989 /sec
1583259429428 cycles # 1.101 GHz (54.57%)
2004002484935 instructions # 1.27 insn per cycle (54.37%)
5594202389 branch-misses (54.36%)
643113574524 cache-references # 447.409 M/sec (54.39%)
18233791870 cache-misses # 2.835 % of all cache refs (54.37%)
640205852062 L1-dcache-loads # 445.386 M/sec (46.75%)
17968160377 L1-dcache-load-misses # 2.81% of all L1-dcache accesses (46.79%)
651747432274 dTLB-loads # 453.415 M/sec (46.59%)
3127124271 dTLB-load-misses # 0.48% of all dTLB cache accesses (46.75%)
535395273064 iTLB-loads # 372.470 M/sec (54.38%)
113500056 iTLB-load-misses # 0.02% of all iTLB cache accesses (54.35%)
628871845924 L1-icache-loads # 437.501 M/sec (46.80%)
22585641203 L1-icache-load-misses # 3.59% of all L1-icache accesses (46.79%)

90.514819303 seconds time elapsed

43.877656000 seconds user
1397.176001000 seconds sys

Link: https://lkml.org/lkml/2021/10/8/598=20
Link: https://lkml.kernel.org/r/20211011144331.70084-1-42.hyeyoo@gmail.com
Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

mm/slub.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/slub.c~mm-slub-use-prefetchw-instead-of-prefetch
+++ a/mm/slub.c
@@ -354,7 +354,7 @@ static inline void *get_freepointer(stru

static void prefetch_freepointer(const struct kmem_cache *s, void *object)
{
- prefetch(object + s->offset);
+ prefetchw(object + s->offset);
}

static inline void *get_freepointer_safe(struct kmem_cache *s, void *object)
_