From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752886AbYE2GWZ (ORCPT ); Thu, 29 May 2008 02:22:25 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751168AbYE2GWS (ORCPT ); Thu, 29 May 2008 02:22:18 -0400 Received: from brick.kernel.dk ([87.55.233.238]:6279 "EHLO kernel.dk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751165AbYE2GWS (ORCPT ); Thu, 29 May 2008 02:22:18 -0400 Date: Thu, 29 May 2008 08:22:15 +0200 From: Jens Axboe To: Andrew Morton Cc: linux-kernel@vger.kernel.org Subject: Re: block: make blktrace use per-cpu buffers for message notes Message-ID: <20080529062214.GG25504@kernel.dk> References: <200805281559.m4SFx7b9022392@hera.kernel.org> <20080528231351.a35001bc.akpm@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20080528231351.a35001bc.akpm@linux-foundation.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 28 2008, Andrew Morton wrote: > On Wed, 28 May 2008 15:59:07 GMT Linux Kernel Mailing List wrote: > > > Gitweb: http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=64565911cdb57c2f512a9715b985b5617402cc67 > > Commit: 64565911cdb57c2f512a9715b985b5617402cc67 > > Parent: 4722dc52a891ab6cb2d637ddb87233e0ce277827 > > Author: Jens Axboe > > AuthorDate: Wed May 28 14:45:33 2008 +0200 > > Committer: Jens Axboe > > CommitDate: Wed May 28 14:49:27 2008 +0200 > > please try to avoid merging unreviewed changes. Just because you didn't review it doesn't mean it's unreviewed :-) It's not unreviewed, it was posted on lkml and a few version were bounced back and forth. > > block: make blktrace use per-cpu buffers for message notes > > > > Currently it uses a single static char array, but that risks > > being corrupted when multiple users issue message notes at the > > same time. Make the buffers dynamically allocated when the trace > > is setup and make them per-cpu instead. > > > > The default max message size of 1k is also very large, the > > interface is mainly for small text notes. So shrink it to 128 bytes. > > > > Signed-off-by: Jens Axboe > > --- > > block/blktrace.c | 15 ++++++++++++--- > > include/linux/blktrace_api.h | 3 ++- > > 2 files changed, 14 insertions(+), 4 deletions(-) > > > > diff --git a/block/blktrace.c b/block/blktrace.c > > index 20e11f3..7ae87cc 100644 > > --- a/block/blktrace.c > > +++ b/block/blktrace.c > > @@ -79,13 +79,16 @@ void __trace_note_message(struct blk_trace *bt, const char *fmt, ...) > > { > > int n; > > va_list args; > > - static char bt_msg_buf[BLK_TN_MAX_MSG]; > > + char *buf; > > > > + preempt_disable(); > > + buf = per_cpu_ptr(bt->msg_data, smp_processor_id()); > > get_cpu()/put_cpu() is the standard way of doing this. Sure, end result is the same though. get_cpu()/put_cpu() is cleaner, though. > > @@ -172,7 +173,7 @@ extern void __trace_note_message(struct blk_trace *, const char *fmt, ...); > > if (unlikely(bt)) \ > > __trace_note_message(bt, fmt, ##__VA_ARGS__); \ > > } while (0) > > -#define BLK_TN_MAX_MSG 1024 > > +#define BLK_TN_MAX_MSG 128 > > It seems a bit strange to do this right when we've taken this _off_ the > stack. But I suppose nothing will break. It was never on the stack, it was a global static char array. We are still allocating memory for this, per-cpu. So I think it still makes sense to shrink the size. It's really meant for small trace messages, 128 bytes is plenty. It's an in-kernel property, the userland app doesn't care. So we could easily grow this in the future, should the need arise. > I cannot work out why 9d5f09a424a67ddb959829894efb4c71cbf6d600 ("Added > in MESSAGE notes for blktraces") was -rc material. Well it's probably really not, but the impact is truly minimal. But strictly speaking, it was not -rcX material, I agree. -- Jens Axboe