Re: [PATCH 5/7] mm/page_alloc: Protect PCP lists with a spinlock

[Nicolas Saenz Julienne <nsaenzju@redhat.com>]

On Fri, 2022-06-24 at 13:54 +0100, Mel Gorman wrote:
> Currently the PCP lists are protected by using local_lock_irqsave to
> prevent migration and IRQ reentrancy but this is inconvenient.  Remote
> draining of the lists is impossible and a workqueue is required and every
> task allocation/free must disable then enable interrupts which is
> expensive.
> 
> As preparation for dealing with both of those problems, protect the lists
> with a spinlock.  The IRQ-unsafe version of the lock is used because IRQs
> are already disabled by local_lock_irqsave.  spin_trylock is used in
> preparation for a time when local_lock could be used instead of
> lock_lock_irqsave.
> 
> The per_cpu_pages still fits within the same number of cache lines after
> this patch relative to before the series.
> 
> struct per_cpu_pages {
>         spinlock_t                 lock;                 /*     0     4 */
>         int                        count;                /*     4     4 */
>         int                        high;                 /*     8     4 */
>         int                        batch;                /*    12     4 */
>         short int                  free_factor;          /*    16     2 */
>         short int                  expire;               /*    18     2 */
> 
>         /* XXX 4 bytes hole, try to pack */
> 
>         struct list_head           lists[13];            /*    24   208 */
> 
>         /* size: 256, cachelines: 4, members: 7 */
>         /* sum members: 228, holes: 1, sum holes: 4 */
>         /* padding: 24 */
> } __attribute__((__aligned__(64)));
> 
> There is overhead in the fast path due to acquiring the spinlock even
> though the spinlock is per-cpu and uncontended in the common case.  Page
> Fault Test (PFT) running on a 1-socket reported the following results on a
> 1 socket machine.
> 
>                                      5.19.0-rc3               5.19.0-rc3
>                                         vanilla      mm-pcpspinirq-v5r16
> Hmean     faults/sec-1   869275.7381 (   0.00%)   874597.5167 *   0.61%*
> Hmean     faults/sec-3  2370266.6681 (   0.00%)  2379802.0362 *   0.40%*
> Hmean     faults/sec-5  2701099.7019 (   0.00%)  2664889.7003 *  -1.34%*
> Hmean     faults/sec-7  3517170.9157 (   0.00%)  3491122.8242 *  -0.74%*
> Hmean     faults/sec-8  3965729.6187 (   0.00%)  3939727.0243 *  -0.66%*
> 
> There is a small hit in the number of faults per second but given that the
> results are more stable, it's borderline noise.
> 
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> ---

Reviewed-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
Tested-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>

Thanks!

-- 
Nicolás Sáenz