Commit 0650b2b2 authored by Ian Rogers's avatar Ian Rogers Committed by Namhyung Kim
Browse files

perf sharded_mutex: Introduce sharded_mutex 



Per object mutexes may come with significant memory cost while a
global mutex can suffer from unnecessary contention. A sharded mutex
is a compromise where objects are hashed and then a particular mutex
for the hash of the object used. Contention can be controlled by the
number of shards.

v2. Use hashmap.h's hash_bits in case of contention from alignment of
    objects.

Signed-off-by: default avatarIan Rogers <irogers@google.com>
Acked-by: default avatarNamhyung Kim <namhyung@kernel.org>
Cc: Andres Freund <andres@anarazel.de>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Yuan Can <yuancan@huawei.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Link: https://lore.kernel.org/r/20230615040715.2064350-1-irogers@google.com


Signed-off-by: default avatarNamhyung Kim <namhyung@kernel.org>
parent 5e37ef5c

tools/perf/util/Build

+1 −0
Original line number Diff line number Diff line
@@ -145,6 +145,7 @@ perf-y += mem2node.o 
perf-y += clockid.o

perf-y += list_sort.o

perf-y += mutex.o

perf-y += sharded_mutex.o



perf-$(CONFIG_LIBBPF) += bpf-loader.o

perf-$(CONFIG_LIBBPF) += bpf_map.o

tools/perf/util/sharded_mutex.c

0 → 100644
+33 −0
Original line number Diff line number Diff line 
// SPDX-License-Identifier: GPL-2.0

#include "sharded_mutex.h"



#include <stdlib.h>



struct sharded_mutex *sharded_mutex__new(size_t num_shards)

{

	struct sharded_mutex *result;

	size_t size;

	unsigned int bits;



	for (bits = 0; ((size_t)1 << bits) < num_shards; bits++)

		;



	size = sizeof(*result) + sizeof(struct mutex) * (1 << bits);

	result = malloc(size);

	if (!result)

		return NULL;



	result->cap_bits = bits;

	for (size_t i = 0; i < ((size_t)1 << bits); i++)

		mutex_init(&result->mutexes[i]);



	return result;

}



void sharded_mutex__delete(struct sharded_mutex *sm)

{

	for (size_t i = 0; i < ((size_t)1 << sm->cap_bits); i++)

		mutex_destroy(&sm->mutexes[i]);



	free(sm);

}

tools/perf/util/sharded_mutex.h

0 → 100644
+29 −0
Original line number Diff line number Diff line 
/* SPDX-License-Identifier: GPL-2.0 */

#ifndef PERF_SHARDED_MUTEX_H

#define PERF_SHARDED_MUTEX_H



#include "mutex.h"

#include "hashmap.h"



/*

 * In a situation where a lock is needed per object, having a mutex can be

 * relatively memory expensive (40 bytes on x86-64). If the object can be

 * constantly hashed, a sharded mutex is an alternative global pool of mutexes

 * where the mutex is looked up from a hash value. This can lead to collisions

 * if the number of shards isn't large enough.

 */

struct sharded_mutex {

	/* mutexes array is 1<<cap_bits in size. */

	unsigned int cap_bits;

	struct mutex mutexes[];

};



struct sharded_mutex *sharded_mutex__new(size_t num_shards);

void sharded_mutex__delete(struct sharded_mutex *sm);



static inline struct mutex *sharded_mutex__get_mutex(struct sharded_mutex *sm, size_t hash)

{

	return &sm->mutexes[hash_bits(hash, sm->cap_bits)];

}



#endif  /* PERF_SHARDED_MUTEX_H */