linux-kernel - Re: [PATCH] xfs: don't allow log recover IO to be throttled

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Message-ID: <67tqyedixro6clydwtp5y2ro6wnlr7fp4sfnpz33l4pf6oy4ir@6yr23tubzsjx>
Date: Mon, 10 Mar 2025 14:17:37 +0100
From: Carlos Maiolino <cem@...nel.org>
To: Jinliang Zheng <alexjlzheng@...il.com>
Cc: david@...morbit.com, alexjlzheng@...cent.com, dchinner@...hat.com, 
	djwong@...nel.org, linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org
Subject: Re: [PATCH] xfs: don't allow log recover IO to be throttled

On Sun, Mar 09, 2025 at 08:41:33PM +0800, Jinliang Zheng wrote:
> On Tue, 4 Mar 2025 07:45:44 +1100, Dave Chinner wrote:
> > On Mon, Mar 03, 2025 at 07:23:01PM +0800, Jinliang Zheng wrote:
> > > When recovering a large filesystem, avoid log recover IO being
> > > throttled by rq_qos_throttle().
> >
> > Why?
> >
> > The only writes to the journal during recovery are to clear stale
> > blocks - it's only a very small part of the IO that journal recovery
> > typically does. What problem happens when these writes are
> > throttled?
> 
> Sorry for the late reply, I was struggling with my work. :-(
> 
> Recently, we encountered the problem of xfs log IO being throttled in
> the Linux distribution version maintained by ourselves. To be more
> precise, it was indirectly throttled by the IO issued by the LVM layer.
> For details, see [1] please.

Ok, so you properly fixed the problem on the DM layer.

> 
> After this problem was solved, we naturally checked other related log
> IO paths, hoping that they would not be throttled by wbt_wait(), that
> is, we hoped that they would be marked with REQ_SYNC | REQ_IDLE.
> 
> For log recover IO, in the LVM scenario, we are not sure whether it
> will be affected by IO on other LVs on the same PV. In addition, we
> did not find any obvious side effects of this patch. An ounce of
> prevention is worth a pound of cure, and we think it is more
> appropriate to add REQ_IDLE here.

If you notice any problem with this that you're trying to fix, or if
this change improves anything, please specify that in the commit message
 - also addressing comments by Christoph, i.e. xfs_rw_bdev shouldn't be
messing with request ops - Just because it has no side-effects is not
a good reason. Regular Log IO being throttled by the DM layer is indeed
a problem, but considering the very small amount of data written here
during log recovery doesn't seem a good use of REQ_IDLE.

So, for now, NAK.

Carlos


> 
> Of course, if there is really a reason not to consider being throttled,
> please forgive me for disturbing you.
> 
> [1] https://lore.kernel.org/linux-xfs/20250220112014.3209940-1-alexjlzheng@tencent.com/
> 
> Thank you very much. :)
> Jinliang Zheng
> 
> >
> > -Dave.
> > --
> > Dave Chinner
> > david@...morbit.com