Re: [PATCH v6 2/2] mm: kick writeback flusher for IOCB_DONTCACHE with targeted dirty tracking

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2026-05-08 at 17:20 -0700, Andrew Morton wrote:
> On Tue, 05 May 2026 20:59:49 +0200 Jeff Layton <jlayton@kernel.org> wrote:
> 
> > The IOCB_DONTCACHE writeback path in generic_write_sync() calls
> > filemap_flush_range() on every write, submitting writeback inline in
> > the writer's context.  Perf lock contention profiling shows the
> > performance problem is not lock contention but the writeback submission
> > work itself — walking the page tree and submitting I/O blocks the writer
> > for milliseconds, inflating p99.9 latency from 23ms (buffered) to 93ms
> > (dontcache).
> > 
> > Replace the inline filemap_flush_range() call with a flusher kick that
> > drains dirty pages in the background.  This moves writeback submission
> > completely off the writer's hot path.
> > 
> > ...
> > 
> >                         Before    After    Change
> >   seq-write/dontcache      298      897    +201%
> >   rand-write/dontcache     131      236     +80%
> > 
> > Tail latency improvements (seq-write/dontcache):
> >   p99:    135,266 us  ->  23,986 us   (-82%)
> >   p99.9: 8,925,479 us ->  28,443 us   (-99.7%)
> > 
> > Multi-writer (4 jobs, sequential write):
> >                                 Before    After    Change
> >   dontcache aggregate (MB/s)     2,529    4,532     +79%
> >   dontcache p99 (us)             8,553    1,002     -88%
> >   dontcache p99.9 (us)         109,314    1,057     -99%
> > 
> > 32-file write (Axboe test):
> >                                 Before    After    Change
> >   dontcache aggregate (MB/s)     1,548    3,499    +126%
> >   dontcache p99 (us)            10,170      602     -94%
> >   Peak dirty pages (MB)          1,837      213     -88%
> > 
> >   Dontcache now reaches 81% of buffered throughput (was 35%).
> > 
> > Competing writers (dontcache vs buffered, separate files):
> >                                 Before    After
> >   buffered writer                  868      433 MB/s
> >   dontcache writer                 415      433 MB/s
> >   Aggregate                      1,284      866 MB/s
> > 
> > ...
> > 
> >   The dontcache writer's p99.9 latency collapsed from 119 ms to
> >   33 ms (-73%), eliminating the severe periodic stalls seen in the
> >   baseline. Both writers now share identical latency profiles,
> >   matching the buffered-vs-buffered pattern.
> > 
> > The per-bdi_writeback dirty tracking dramatically reduces peak dirty
> > pages in dontcache workloads, with the 32-file test dropping from
> > 1.8 GB to 213 MB. Dontcache sequential write throughput triples and
> > multi-writer throughput reaches parity with buffered I/O, with tail
> > latencies collapsing by 1-2 orders of magnitude.
> 
> Geeze, is that the best you can do ;)
> 
> Sashiko seems to have found more stuff:
> 	https://sashiko.dev/#/patchset/20260505-dontcache-v6-0-66463805dd6a@kernel.org

I saw those after I sent the last set and have been working on
addressing them. I've also found a couple more via dueling Gemini and
Claude reviews.

I'll have a v7 posting coming early next week.

Thanks,
-- 
Jeff Layton <jlayton@kernel.org>