Messages in this thread | | | Date | Thu, 29 May 2008 08:45:20 +0200 | From | Jens Axboe <> | 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@oracle.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@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. > > 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
| |