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SubjectRe: [PATCH v2] mm: slowly shrink slabs with a relatively small number of objects
Hi Roman,

Thank you for the patch! Perhaps something to improve:

[auto build test WARNING on linus/master]
[also build test WARNING on v4.19-rc2 next-20180905]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url: https://github.com/0day-ci/linux/commits/Roman-Gushchin/mm-slowly-shrink-slabs-with-a-relatively-small-number-of-objects/20180906-142351
config: openrisc-or1ksim_defconfig (attached as .config)
compiler: or1k-linux-gcc (GCC) 6.0.0 20160327 (experimental)
reproduce:
wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
chmod +x ~/bin/make.cross
# save the attached .config to linux build tree
make.cross ARCH=openrisc

All warnings (new ones prefixed by >>):

In file included from include/asm-generic/bug.h:18:0,
from ./arch/openrisc/include/generated/asm/bug.h:1,
from include/linux/bug.h:5,
from include/linux/mmdebug.h:5,
from include/linux/mm.h:9,
from mm/vmscan.c:17:
mm/vmscan.c: In function 'do_shrink_slab':
include/linux/kernel.h:845:29: warning: comparison of distinct pointer types lacks a cast
(!!(sizeof((typeof(x) *)1 == (typeof(y) *)1)))
^
include/linux/kernel.h:859:4: note: in expansion of macro '__typecheck'
(__typecheck(x, y) && __no_side_effects(x, y))
^~~~~~~~~~~
include/linux/kernel.h:869:24: note: in expansion of macro '__safe_cmp'
__builtin_choose_expr(__safe_cmp(x, y), \
^~~~~~~~~~
include/linux/kernel.h:885:19: note: in expansion of macro '__careful_cmp'
#define max(x, y) __careful_cmp(x, y, >)
^~~~~~~~~~~~~
>> mm/vmscan.c:488:10: note: in expansion of macro 'max'
delta = max(delta, min(freeable, batch_size));
^~~

vim +/max +488 mm/vmscan.c

446
447 static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
448 struct shrinker *shrinker, int priority)
449 {
450 unsigned long freed = 0;
451 unsigned long long delta;
452 long total_scan;
453 long freeable;
454 long nr;
455 long new_nr;
456 int nid = shrinkctl->nid;
457 long batch_size = shrinker->batch ? shrinker->batch
458 : SHRINK_BATCH;
459 long scanned = 0, next_deferred;
460
461 if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
462 nid = 0;
463
464 freeable = shrinker->count_objects(shrinker, shrinkctl);
465 if (freeable == 0 || freeable == SHRINK_EMPTY)
466 return freeable;
467
468 /*
469 * copy the current shrinker scan count into a local variable
470 * and zero it so that other concurrent shrinker invocations
471 * don't also do this scanning work.
472 */
473 nr = atomic_long_xchg(&shrinker->nr_deferred[nid], 0);
474
475 total_scan = nr;
476 delta = freeable >> priority;
477 delta *= 4;
478 do_div(delta, shrinker->seeks);
479
480 /*
481 * Make sure we apply some minimal pressure even on
482 * small cgroups. This is necessary because some of
483 * belonging objects can hold a reference to a dying
484 * child cgroup. If we don't scan them, the dying
485 * cgroup can't go away unless the memory pressure
486 * (and the scanning priority) raise significantly.
487 */
> 488 delta = max(delta, min(freeable, batch_size));
489
490 total_scan += delta;
491 if (total_scan < 0) {
492 pr_err("shrink_slab: %pF negative objects to delete nr=%ld\n",
493 shrinker->scan_objects, total_scan);
494 total_scan = freeable;
495 next_deferred = nr;
496 } else
497 next_deferred = total_scan;
498
499 /*
500 * We need to avoid excessive windup on filesystem shrinkers
501 * due to large numbers of GFP_NOFS allocations causing the
502 * shrinkers to return -1 all the time. This results in a large
503 * nr being built up so when a shrink that can do some work
504 * comes along it empties the entire cache due to nr >>>
505 * freeable. This is bad for sustaining a working set in
506 * memory.
507 *
508 * Hence only allow the shrinker to scan the entire cache when
509 * a large delta change is calculated directly.
510 */
511 if (delta < freeable / 4)
512 total_scan = min(total_scan, freeable / 2);
513
514 /*
515 * Avoid risking looping forever due to too large nr value:
516 * never try to free more than twice the estimate number of
517 * freeable entries.
518 */
519 if (total_scan > freeable * 2)
520 total_scan = freeable * 2;
521
522 trace_mm_shrink_slab_start(shrinker, shrinkctl, nr,
523 freeable, delta, total_scan, priority);
524
525 /*
526 * Normally, we should not scan less than batch_size objects in one
527 * pass to avoid too frequent shrinker calls, but if the slab has less
528 * than batch_size objects in total and we are really tight on memory,
529 * we will try to reclaim all available objects, otherwise we can end
530 * up failing allocations although there are plenty of reclaimable
531 * objects spread over several slabs with usage less than the
532 * batch_size.
533 *
534 * We detect the "tight on memory" situations by looking at the total
535 * number of objects we want to scan (total_scan). If it is greater
536 * than the total number of objects on slab (freeable), we must be
537 * scanning at high prio and therefore should try to reclaim as much as
538 * possible.
539 */
540 while (total_scan >= batch_size ||
541 total_scan >= freeable) {
542 unsigned long ret;
543 unsigned long nr_to_scan = min(batch_size, total_scan);
544
545 shrinkctl->nr_to_scan = nr_to_scan;
546 shrinkctl->nr_scanned = nr_to_scan;
547 ret = shrinker->scan_objects(shrinker, shrinkctl);
548 if (ret == SHRINK_STOP)
549 break;
550 freed += ret;
551
552 count_vm_events(SLABS_SCANNED, shrinkctl->nr_scanned);
553 total_scan -= shrinkctl->nr_scanned;
554 scanned += shrinkctl->nr_scanned;
555
556 cond_resched();
557 }
558
559 if (next_deferred >= scanned)
560 next_deferred -= scanned;
561 else
562 next_deferred = 0;
563 /*
564 * move the unused scan count back into the shrinker in a
565 * manner that handles concurrent updates. If we exhausted the
566 * scan, there is no need to do an update.
567 */
568 if (next_deferred > 0)
569 new_nr = atomic_long_add_return(next_deferred,
570 &shrinker->nr_deferred[nid]);
571 else
572 new_nr = atomic_long_read(&shrinker->nr_deferred[nid]);
573
574 trace_mm_shrink_slab_end(shrinker, nid, freed, nr, new_nr, total_scan);
575 return freed;
576 }
577

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