Re: [PATCH 1/3] mm, meminit: Recalculate pcpu batch and high limits after init completes

[Qian Cai <cai@lca.pw>]

> On Oct 21, 2019, at 5:48 AM, Mel Gorman <mgorman@techsingularity.net> wrote:
> 
> Deferred memory initialisation updates zone->managed_pages during
> the initialisation phase but before that finishes, the per-cpu page
> allocator (pcpu) calculates the number of pages allocated/freed in
> batches as well as the maximum number of pages allowed on a per-cpu list.
> As zone->managed_pages is not up to date yet, the pcpu initialisation
> calculates inappropriately low batch and high values.
> 
> This increases zone lock contention quite severely in some cases with the
> degree of severity depending on how many CPUs share a local zone and the
> size of the zone. A private report indicated that kernel build times were
> excessive with extremely high system CPU usage. A perf profile indicated
> that a large chunk of time was lost on zone->lock contention.
> 
> This patch recalculates the pcpu batch and high values after deferred
> initialisation completes for every populated zone in the system. It
> was tested on a 2-socket AMD EPYC 2 machine using a kernel compilation
> workload -- allmodconfig and all available CPUs.
> 
> mmtests configuration: config-workload-kernbench-max
> Configuration was modified to build on a fresh XFS partition.
> 
> kernbench
>                                5.4.0-rc3              5.4.0-rc3
>                                  vanilla           resetpcpu-v2
> Amean     user-256    13249.50 (   0.00%)    16401.31 * -23.79%*
> Amean     syst-256    14760.30 (   0.00%)     4448.39 *  69.86%*
> Amean     elsp-256      162.42 (   0.00%)      119.13 *  26.65%*
> Stddev    user-256       42.97 (   0.00%)       19.15 (  55.43%)
> Stddev    syst-256      336.87 (   0.00%)        6.71 (  98.01%)
> Stddev    elsp-256        2.46 (   0.00%)        0.39 (  84.03%)
> 
>                   5.4.0-rc3    5.4.0-rc3
>                     vanilla resetpcpu-v2
> Duration User       39766.24     49221.79
> Duration System     44298.10     13361.67
> Duration Elapsed      519.11       388.87
> 
> The patch reduces system CPU usage by 69.86% and total build time by
> 26.65%. The variance of system CPU usage is also much reduced.
> 
> Before, this was the breakdown of batch and high values over all zones was.
> 
>    256               batch: 1
>    256               batch: 63
>    512               batch: 7
>    256               high:  0
>    256               high:  378
>    512               high:  42
> 
> 512 pcpu pagesets had a batch limit of 7 and a high limit of 42. After the patch
> 
>    256               batch: 1
>    768               batch: 63
>    256               high:  0
>    768               high:  378
> 
> Cc: stable@vger.kernel.org # v4.1+
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> ---
> mm/page_alloc.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index c0b2e0306720..f972076d0f6b 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1947,6 +1947,14 @@ void __init page_alloc_init_late(void)
> 	/* Block until all are initialised */
> 	wait_for_completion(&pgdat_init_all_done_comp);
> 
> +	/*
> +	 * The number of managed pages has changed due to the initialisation
> +	 * so the pcpu batch and high limits needs to be updated or the limits
> +	 * will be artificially small.
> +	 */
> +	for_each_populated_zone(zone)
> +		zone_pcp_update(zone);
> +
> 	/*
> 	 * We initialized the rest of the deferred pages.  Permanently disable
> 	 * on-demand struct page initialization.
> -- 
> 2.16.4
> 
> 

Warnings from linux-next,

[   14.265911][  T659] BUG: sleeping function called from invalid context at kernel/locking/mutex.c:935
[   14.265992][  T659] in_atomic(): 1, irqs_disabled(): 1, non_block: 0, pid: 659, name: pgdatinit8
[   14.266044][  T659] 1 lock held by pgdatinit8/659:
[   14.266075][  T659]  #0: c000201ffca87b40 (&(&pgdat->node_size_lock)->rlock){....}, at: deferred_init_memmap+0xc4/0x26c
[   14.266160][  T659] irq event stamp: 26
[   14.266194][  T659] hardirqs last  enabled at (25): [<c000000000950584>] _raw_spin_unlock_irq+0x44/0x80
[   14.266246][  T659] hardirqs last disabled at (26): [<c0000000009502ec>] _raw_spin_lock_irqsave+0x3c/0xa0
[   14.266299][  T659] softirqs last  enabled at (0): [<c0000000000ff8d0>] copy_process+0x720/0x19b0
[   14.266339][  T659] softirqs last disabled at (0): [<0000000000000000>] 0x0
[   14.266400][  T659] CPU: 64 PID: 659 Comm: pgdatinit8 Not tainted 5.4.0-rc4-next-20191021 #1
[   14.266462][  T659] Call Trace:
[   14.266494][  T659] [c00000003d8efae0] [c000000000921cf4] dump_stack+0xe8/0x164 (unreliable)
[   14.266538][  T659] [c00000003d8efb30] [c000000000157c54] ___might_sleep+0x334/0x370
[   14.266577][  T659] [c00000003d8efbb0] [c00000000094a784] __mutex_lock+0x84/0xb20
[   14.266627][  T659] [c00000003d8efcc0] [c000000000954038] zone_pcp_update+0x34/0x64
[   14.266677][  T659] [c00000003d8efcf0] [c000000000b9e6bc] deferred_init_memmap+0x1b8/0x26c
[   14.266740][  T659] [c00000003d8efdb0] [c000000000149528] kthread+0x1a8/0x1b0
[   14.266792][  T659] [c00000003d8efe20] [c00000000000b748] ret_from_kernel_thread+0x5c/0x74
[   14.268288][  T659] node 8 initialised, 1879186 pages in 12200ms
[   14.268527][  T659] pgdatinit8 (659) used greatest stack depth: 27984 bytes left
[   15.589983][  T658] BUG: sleeping function called from invalid context at kernel/locking/mutex.c:935
[   15.590041][  T658] in_atomic(): 1, irqs_disabled(): 1, non_block: 0, pid: 658, name: pgdatinit0
[   15.590078][  T658] 1 lock held by pgdatinit0/658:
[   15.590108][  T658]  #0: c000001fff5c7b40 (&(&pgdat->node_size_lock)->rlock){....}, at: deferred_init_memmap+0xc4/0x26c
[   15.590192][  T658] irq event stamp: 18
[   15.590224][  T658] hardirqs last  enabled at (17): [<c000000000950654>] _raw_spin_unlock_irqrestore+0x94/0xd0
[   15.590283][  T658] hardirqs last disabled at (18): [<c0000000009502ec>] _raw_spin_lock_irqsave+0x3c/0xa0
[   15.590332][  T658] softirqs last  enabled at (0): [<c0000000000ff8d0>] copy_process+0x720/0x19b0
[   15.590379][  T658] softirqs last disabled at (0): [<0000000000000000>] 0x0
[   15.590414][  T658] CPU: 8 PID: 658 Comm: pgdatinit0 Tainted: G        W         5.4.0-rc4-next-20191021 #1
[   15.590460][  T658] Call Trace:
[   15.590491][  T658] [c00000003d8cfae0] [c000000000921cf4] dump_stack+0xe8/0x164 (unreliable)
[   15.590541][  T658] [c00000003d8cfb30] [c000000000157c54] ___might_sleep+0x334/0x370
[   15.590588][  T658] [c00000003d8cfbb0] [c00000000094a784] __mutex_lock+0x84/0xb20
[   15.590643][  T658] [c00000003d8cfcc0] [c000000000954038] zone_pcp_update+0x34/0x64
[   15.590689][  T658] [c00000003d8cfcf0] [c000000000b9e6bc] deferred_init_memmap+0x1b8/0x26c
[   15.590739][  T658] [c00000003d8cfdb0] [c000000000149528] kthread+0x1a8/0x1b0
[   15.590790][  T658] [c00000003d8cfe20] [c00000000000b748] ret_from_kernel_thread+0x5c/0x74