linux-kernel - Re: block: make blktrace use per-cpu buffers for message notes

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Message-ID: <20080529064520.GJ25504@kernel.dk>
Date:	Thu, 29 May 2008 08:45:20 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: block: make blktrace use per-cpu buffers for message notes

On Wed, May 28 2008, Andrew Morton wrote:
> On Thu, 29 May 2008 08:22:15 +0200 Jens Axboe <jens.axboe@...cle.com> wrote:
> 
> > On Wed, May 28 2008, Andrew Morton wrote:
> > > On Wed, 28 May 2008 15:59:07 GMT Linux Kernel Mailing List <linux-kernel@...r.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@...cle.com>
> > > > AuthorDate: Wed May 28 14:45:33 2008 +0200
> > > > Committer:  Jens Axboe <jens.axboe@...cle.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.
> 
> OK.  The Subject: swizzling confounded me.
> 
> > > >  		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.
> 
> yup.
> 
> It's a bit sad to stage the data in a local per-cpu buffer and then
> copy it into relay's per-cpu buffer.  I guess this is because the
> length of the output isn't known beforehand.  Could be fixed by doing
> what kvasprintf() does, but that might well be slower.

I agree, this is what we debated. My reasoning is that it's better
to minimize usage of the relay buffer, so the stage-and-copy doesn't
matter a whole lot.

I seem to recall a relay_unreserve() patch from Tom back in the day,
if we had something like that we could do the optimal approach of:

        buf = relay_reserver(max_size);
        n = vscnprintf(buf, max_size, ...);
        if (max_size - n)
                relay_unreserve(max_size - n);

and get the best of both worlds. But, again, it's not really a big deal
I think.

My main interest in this is adding cfq trace messages, so we have
direct ways of comparing queue+dispatch with what cfq is deciding to
do.

-- 
Jens Axboe

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