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@xxxxxxxxxxxxxxxxxxx>
> ---
Reviewed-by: Nicolas Saenz Julienne <nsaenzju@xxxxxxxxxx>
Tested-by: Nicolas Saenz Julienne <nsaenzju@xxxxxxxxxx>
Thanks!
--
Nicolás Sáenz
