Re: [PATCH] iomap: Fix the write_count in iomap_add_to_ioend().

[Darrick J. Wong]

On Thu, Sep 17, 2020 at 03:48:04PM +0100, Christoph Hellwig wrote:
> On Thu, Sep 17, 2020 at 06:42:19AM -0400, Brian Foster wrote:
> > That wouldn't address the latency concern Dave brought up. That said, I
> > have no issue with this as a targeted solution for the softlockup issue.
> > iomap_finish_ioend[s]() is common code for both the workqueue and
> > ->bi_end_io() contexts so that would require either some kind of context
> > detection (and my understanding is in_atomic() is unreliable/frowned
> > upon) or a new "atomic" parameter through iomap_finish_ioend[s]() to
> > indicate whether it's safe to reschedule. Preference?
> 
> True, it would not help with latency.  But then again the latency
> should be controlled by the writeback code not doing giant writebacks
> to start with, shouldn't it?
> 
> Any XFS/iomap specific limit also would not help with the block layer
> merging bios.

/me hasn't totally been following this thread, but iomap will also
aggregate the ioend completions; do we need to cap that to keep
latencies down?  I was assuming that amortization was always favorable,
but maybe not?

--D