linux-kernel - Re: [PATCH 2/3] relay: Fix race condition which occurs when reading across CPUs.

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Date:	Tue, 17 Jun 2008 15:49:23 +0300
From:	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>
To:	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>
Cc:	Jens Axboe <jens.axboe@...cle.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	Tom Zanussi <tzanussi@...il.com>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, righi.andrea@...il.com
Subject: Re: [PATCH 2/3] relay: Fix race condition which occurs when reading
 across CPUs.

On Tue, 17 Jun 2008 15:39:34 +0300
Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro> wrote:

> kmemtrace will use the affine versions and set CPU affinity anyway,
> but it would be nice to have a consistent behavior from relay's part.
> 
> 
> 	Cheers,
> 	Eduard

BTW, I wrote this stuff and tested affinity behavior. It seems like
migration always occurs fast enough, even if under pressure. My machine
has two CPUs, so there are a few hardcoded values.

#define _GNU_SOURCE

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <string.h>
#include <sched.h>

unsigned long get_current_cpu(FILE *stat_fp)
{
	char buf[256], *p;
	unsigned long n_delims = 0, ret;
	size_t count;
	
	fseek(stat_fp, 0, SEEK_SET);
	count = fread(buf, 1, 255, stat_fp);
	buf[count] = 0;

	p = buf;
	while (n_delims < 38)
		if (*p++ == ' ')
			n_delims++;
	sscanf(p, "%lu", &ret);

	return ret;
}

int main(void)
{
	FILE *stat_fp;
	char stat_filename[255];
	int err;
	unsigned long old_cpu, new_cpu, cpu;
	pid_t pid;
	cpu_set_t cpuset;

	pid = getpid();
	sprintf(stat_filename, "/proc/%lu/stat", pid);
	stat_fp = fopen(stat_filename, "r");
	if (!stat_fp) {
		printf("Couldn't open %lu", pid);
		return 1;
	}

	old_cpu = get_current_cpu(stat_fp);
	new_cpu = (old_cpu + 1) % 2;

	printf("Started on CPU %lu, PID %lu, setting affinity to CPU %lu...\n",
	       old_cpu, pid, new_cpu);
	
	err = sched_getaffinity(pid, sizeof(cpu_set_t), &cpuset);
	if (err == -1){
		printf("Could not retrieve the affinity!\n");
		return 1;
	}
	printf("Previous affinity: %d %d\n",
	       CPU_ISSET(0, &cpuset), CPU_ISSET(1, &cpuset));
	CPU_ZERO(&cpuset);
	CPU_SET(new_cpu, &cpuset);
	err = sched_setaffinity(pid, sizeof(cpu_set_t), &cpuset);
	if (err == -1) {
		printf("Could not set affinity!\n");
		return 1;
	}
	err = sched_getaffinity(pid, sizeof(cpu_set_t), &cpuset);
	if (err == -1){
		printf("Could not retrieve the affinity!\n");
		return 1;
	}
	printf("Current affinity: %d %d\n",
	       CPU_ISSET(0, &cpuset), CPU_ISSET(1, &cpuset));

	cpu = get_current_cpu(stat_fp);
	if (cpu != new_cpu) {
		printf("Process migration did not occur immediately!\n"
		       "--- we were supposed to be on %lu, but we're on %lu\n",
		       new_cpu, cpu);
		/* return 1; */
	}

	for (;;) {
		cpu = get_current_cpu(stat_fp);
		if (cpu != new_cpu) {
			printf("Oh, we arrived on a different CPU!\n");
			/* return 1; */
		}
	}

	return 0;
}

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