Re: [PATCH v4] mm/page_alloc: avoid direct compaction for costly __GFP_NORETRY allocations

[Salvatore Dipietro]

On 9/19/26 06:13, Matthew Wilcox wrote:
> This patch is still piling hack on hack.  We haven't made a serious
> effort to understand what's going on, we're just adjusting flags until
> things stop sucking.

I have tested two new kernel patches on v7.3-rc1, each isolating one behaviour
(diffs at the bottom), and ran the same pgbench simple-update workload
(1024 clients / 96 threads / 1200s, 3 iterations each).  Baseline is the
unpatched regressed kernel; target is ~135k, the pre-5d8edfb900d5 number.

  Config                                          Avg TPS   % vs baseline
  v7.3-rc1 baseline (no patch)                      59,408            -
  (a) post-reclaim drain step skipped               70,615       +18.9%
  (b) compaction disabled only                     107,210       +80.5%
  (c) v4 (full non-blocking, reference)            154,835      +160.6%


We can notice that:

1. The drain_all_pages() IPI is not the driver.  (a) skips the whole
   post-reclaim drain step for costly __GFP_NORETRY -- that is
   unreserve_highatomic_pageblock(), drain_all_pages() and the retry
   together -- and the entire step is worth only ~11k of the ~96k gap.
   Whatever the split between the three, the cross-CPU drain cannot
   account for the bulk of it.

2. Turning compaction off is not enough.  Your gfp_compaction_allowed()
   one-liner (b) gets about half the gap, and it declines across
   iterations (135k -> 98k -> 89k).  It drops compaction for
   __GFP_NORETRY at every order that can use it, __GFP_THISNODE
   excepted, not only at the costly order v4 gates on; the decline is
   consistent with the zone no longer being repaired.  (c) only makes
   the costly attempt non-blocking and leaves kswapd/kcompactd working,
   and it does not show the decline.

3. We have collected metrics to understand how often the
   allocator stalls and normalised to a million page writebacks (nr_written),
   over the same 3 x 1200s iterations as above:

                                        compact_  allocstall_   pgscan_
                                           stall      movable    direct
     v7.3-rc1 baseline                       513          277    90,230
     (a) post-reclaim drain skipped          912          472   129,249
     (b) compaction disabled                   0          778    44,737
     (c) v4 (full non-blocking)                5           39     4,407

   The baseline enters stall compaction ~100x more often per page written
   than (c) does.  (a) makes all three metrics worse.  (b) removes direct
   compaction entirely (0 stalls) but does not remove the work: it enters
   direct reclaim 2.8x more often than the baseline (778 vs 277
   allocstall_movable), and still does half the baseline's direct scanning
   (44,737 vs 90,230 pgscan_direct).  (c) cuts both instead -- reclaim
   stalls 7x lower and direct scanning 20x lower (39 and 4,407).
   That is why (b) recovers only half the gap -- the stall moves from
   compaction into reclaim instead of going away.

If you agree this is the right direction, I am happy to send a v6 with
(c)'s behaviour plus the defrag_mode fix discussed in the v5 thread.


Thanks,
Salvatore


---
For reproducibility, here is the exact diff behind each measured row
above (all against v7.3-rc1).

(a) post-reclaim drain step skipped -- 70,615 tps:

  diff --git a/mm/page_alloc.c b/mm/page_alloc.c
  --- a/mm/page_alloc.c
  +++ b/mm/page_alloc.c
  @@ -4487,7 +4487,8 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
   	 * pages are pinned on the per-cpu lists or in high alloc reserves.
   	 * Shrink them and try again
   	 */
  -	if (!page && !drained) {
  +	if (!page && !drained &&
  +	    !(order > PAGE_ALLOC_COSTLY_ORDER && (gfp_mask & __GFP_NORETRY))) {
   		unreserve_highatomic_pageblock(ac, false);
   		drain_all_pages(NULL);
   		drained = true;

(b) compaction disabled only -- 107,210 tps:

  diff --git a/include/linux/gfp.h b/include/linux/gfp.h
  --- a/include/linux/gfp.h
  +++ b/include/linux/gfp.h
  @@ -380,7 +380,8 @@ static inline bool gfp_has_io_fs(gfp_t gfp)
    */
   static inline bool gfp_compaction_allowed(gfp_t gfp_mask)
   {
  -	return IS_ENABLED(CONFIG_COMPACTION) && (gfp_mask & __GFP_IO);
  +	return IS_ENABLED(CONFIG_COMPACTION) && (gfp_mask & __GFP_IO) &&
  +		(!(gfp_mask & __GFP_NORETRY) || (gfp_mask & __GFP_THISNODE));
   }



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