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SubjectRe: [PATCH v3] mm: fix tick timer stall during deferred page init
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On 11.03.20 13:38, Shile Zhang wrote:
> When 'CONFIG_DEFERRED_STRUCT_PAGE_INIT' is set, 'pgdatinit' kthread will
> initialise the deferred pages with local interrupts disabled. It is
> introduced by commit 3a2d7fa8a3d5 ("mm: disable interrupts while
> initializing deferred pages").
>
> On machine with NCPUS <= 2, the 'pgdatinit' kthread could be bound to
> the boot CPU, which could caused the tick timer long time stall, system
> jiffies not be updated in time.
>
> The dmesg shown that:
>
> [ 0.197975] node 0 initialised, 32170688 pages in 1ms
>
> Obviously, 1ms is unreasonable.
>
> Now, fix it by restore in the pending interrupts for every 32*1204 pages
> (128MB) initialized, give the chance to update the systemd jiffies.
> The reasonable demsg shown likes:
>
> [ 1.069306] node 0 initialised, 32203456 pages in 894ms
>
> Fixes: 3a2d7fa8a3d5 ("mm: disable interrupts while initializing deferred pages").
>
> Co-developed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
> Signed-off-by: Shile Zhang <shile.zhang@linux.alibaba.com>
> ---
> mm/page_alloc.c | 25 ++++++++++++++++++++++---
> 1 file changed, 22 insertions(+), 3 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3c4eb750a199..a3a47845e150 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1763,12 +1763,17 @@ deferred_init_maxorder(u64 *i, struct zone *zone, unsigned long *start_pfn,
> return nr_pages;
> }
>
> +/*
> + * Release the pending interrupts for every TICK_PAGE_COUNT pages.
> + */
> +#define TICK_PAGE_COUNT (32 * 1024)
> +
> /* Initialise remaining memory on a node */
> static int __init deferred_init_memmap(void *data)
> {
> pg_data_t *pgdat = data;
> const struct cpumask *cpumask = cpumask_of_node(pgdat->node_id);
> - unsigned long spfn = 0, epfn = 0, nr_pages = 0;
> + unsigned long spfn = 0, epfn = 0, nr_pages = 0, prev_nr_pages = 0;
> unsigned long first_init_pfn, flags;
> unsigned long start = jiffies;
> struct zone *zone;
> @@ -1779,6 +1784,7 @@ static int __init deferred_init_memmap(void *data)
> if (!cpumask_empty(cpumask))
> set_cpus_allowed_ptr(current, cpumask);
>
> +again:
> pgdat_resize_lock(pgdat, &flags);
> first_init_pfn = pgdat->first_deferred_pfn;
> if (first_init_pfn == ULONG_MAX) {
> @@ -1790,7 +1796,6 @@ static int __init deferred_init_memmap(void *data)
> /* Sanity check boundaries */
> BUG_ON(pgdat->first_deferred_pfn < pgdat->node_start_pfn);
> BUG_ON(pgdat->first_deferred_pfn > pgdat_end_pfn(pgdat));
> - pgdat->first_deferred_pfn = ULONG_MAX;
>
> /* Only the highest zone is deferred so find it */
> for (zid = 0; zid < MAX_NR_ZONES; zid++) {
> @@ -1809,9 +1814,23 @@ static int __init deferred_init_memmap(void *data)
> * that we can avoid introducing any issues with the buddy
> * allocator.
> */
> - while (spfn < epfn)
> + while (spfn < epfn) {
> nr_pages += deferred_init_maxorder(&i, zone, &spfn, &epfn);
> + /*
> + * Release the interrupts for every TICK_PAGE_COUNT pages
> + * (128MB) to give tick timer the chance to update the

Nit: 128MB is only true for 4k pages.

> + * system jiffies.
> + */
> + if ((nr_pages - prev_nr_pages) > TICK_PAGE_COUNT) {
> + prev_nr_pages = nr_pages;
> + pgdat->first_deferred_pfn = spfn;
> + pgdat_resize_unlock(pgdat, &flags);
> + goto again;
> + }
> + }
> +

I find that deferred page init code horribly complicated to understand,
but that's a different story.

Your change looks sane to me and survives my basic testing. Thanks!

Acked-by: David Hildenbrand <david@redhat.com>

--
Thanks,

David / dhildenb

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