Messages in this thread | | | Date | Thu, 29 May 2008 08:22:15 +0200 | From | Jens Axboe <> | Subject | Re: block: make blktrace use per-cpu buffers for message notes |
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On Wed, May 28 2008, Andrew Morton wrote: > On Wed, 28 May 2008 15:59:07 GMT Linux Kernel Mailing List <linux-kernel@vger.kernel.org> 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 <jens.axboe@oracle.com> > > AuthorDate: Wed May 28 14:45:33 2008 +0200 > > Committer: Jens Axboe <jens.axboe@oracle.com> > > 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 <jens.axboe@oracle.com> > > --- > > 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
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