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Date: Wed, 15 Jul 2015 21:08:15 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Daniel Wagner <daniel.wagner@bmw-carit.de>,
        Davidlohr Bueso <dave@stgolabs.net>, Ingo Molnar <mingo@redhat.com>,
        Tejun Heo <tj@kernel.org>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/7] rcu: Create rcu_sync infrastructure
Message-ID: <20150715190815.GC2101@redhat.com>
References: <20150711233535.GA829@redhat.com> <20150711233548.GA843@redhat.com> <20150715180546.GJ3717@linux.vnet.ibm.com> <20150715181511.GT19282@twins.programming.kicks-ass.net> <20150715182801.GM3717@linux.vnet.ibm.com>
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On 07/15, Paul E. McKenney wrote:
>
> On Wed, Jul 15, 2015 at 08:15:11PM +0200, Peter Zijlstra wrote:
> >
> > No, it makes the read-side primitive contain an unconditional memory
> > barrier, that forgoes the entire point.
> >
> > The writers are stupidly expensive already for they need global
> > serialization, optimizing them in any way doesn't make sense.
>
> That could well be the case, but it would be good to see the numbers.

Please see the discussion in another "change sb_writers to use
percpu_rw_semaphore".

The simple test-case from Dave

	#include <fcntl.h>
	#include <stdlib.h>
	#include <unistd.h>
	#include <string.h>
	#include <assert.h>

	#define BUFLEN 1
	#define FILESIZE (1 * 1024 * 1024)

	char *testcase_description = "Separate file write";

	void testcase(unsigned long long *iterations)
	{
		char buf[BUFLEN];
		char tmpfile[] = "/run/user/1000/willitscale.XXXXXX";
		int fd = mkstemp(tmpfile);
		unsigned long size = 0;

		memset(buf, 0, sizeof(buf));
		assert(fd >= 0);
		unlink(tmpfile);

		while (1) {
			int ret = write(fd, buf, BUFLEN);
			assert(ret >= 0);
			size += ret;
			if (size >= FILESIZE) {
				size = 0;
				lseek(fd, 0, SEEK_SET);
			}

			(*iterations)++;
		}
	}

runs 12% faster if we "simply" remove mb's from sb_start/end_write().
percpu_rw_semaphore does this too and has the approximately same
performance, and we can (hopefully) remove this nontrivial, currently
not 100% correct, and very "special" code in fs/super.c.

Oleg.