Hi David
Thanks for you quickly reply.
On 22/07/2024 14:44, Vlastimil Babka (SUSE) wrote:
> On 7/22/24 4:10 AM, Li Zhijian wrote:
>> It's expected that no page should be left in pcp_list after calling
>> zone_pcp_disable() in offline_pages(). Previously, it's observed that
>> offline_pages() gets stuck [1] due to some pages remaining in pcp_list.
>>
>> Cause:
>> There is a race condition between drain_pages_zone() and __rmqueue_pcplist()
>> involving the pcp->count variable. See below scenario:
>>
>> CPU0 CPU1
>> ---------------- ---------------
>> spin_lock(&pcp->lock);
>> __rmqueue_pcplist() {
>> zone_pcp_disable() {
>> /* list is empty */
>> if (list_empty(list)) {
>> /* add pages to pcp_list */
>> alloced = rmqueue_bulk()
>> mutex_lock(&pcp_batch_high_lock)
>> ...
>> __drain_all_pages() {
>> drain_pages_zone() {
>> /* read pcp->count, it's 0 here */
>> count = READ_ONCE(pcp->count)
>> /* 0 means nothing to drain */
>> /* update pcp->count */
>> pcp->count += alloced << order;
>> ...
>> ...
>> spin_unlock(&pcp->lock);
>>
>> In this case, after calling zone_pcp_disable() though, there are still some
>> pages in pcp_list. And these pages in pcp_list are neither movable nor
>> isolated, offline_pages() gets stuck as a result.
>>
>> Solution:
>> Expand the scope of the pcp->lock to also protect pcp->count in
>> drain_pages_zone(), to ensure no pages are left in the pcp list after
>> zone_pcp_disable()
>>
>> [1] https://lore.kernel.org/linux-mm/6a07125f-e720-404c-b2f9-e55f3f166e85@xxxxxxxxxxx/
>>
>> Cc: David Hildenbrand <david@xxxxxxxxxx>
>> Cc: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>
>> Reported-by: Yao Xingtao <yaoxt.fnst@xxxxxxxxxxx>
>> Signed-off-by: Li Zhijian <lizhijian@xxxxxxxxxxx>
>
> Can we find a breaking commit for Fixes: ?
I haven't confirmed the FBC because my reproducer is not fit to run in the old kernel for some reasons.
but I noticed it didn't read the count without lock held since below commit
4b23a68f9536 mm/page_alloc: protect PCP lists with a spinlock
>
>> ---
>> V2:
>> - Narrow down the scope of the spin_lock() to limit the draining latency. # Vlastimil and David
>> - In above scenario, it's sufficient to read pcp->count once with lock held, and it fully fixed
>> my issue[1] in thounds runs(It happened in more than 5% before).
>
> That should be ok indeed, but...
>
>> RFC:
>> https://lore.kernel.org/linux-mm/20240716073929.843277-1-lizhijian@xxxxxxxxxxx/
>> ---
>> mm/page_alloc.c | 5 ++++-
>> 1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 9ecf99190ea2..5388a35c4e9c 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -2323,8 +2323,11 @@ void drain_zone_pages(struct zone *zone, struct per_cpu_pages *pcp)
>> static void drain_pages_zone(unsigned int cpu, struct zone *zone)
>> {
>> struct per_cpu_pages *pcp = per_cpu_ptr(zone->per_cpu_pageset, cpu);
>> - int count = READ_ONCE(pcp->count);
>> + int count;
>>
>> + spin_lock(&pcp->lock);
>> + count = pcp->count;
>> + spin_unlock(&pcp->lock);
>> while (count) {
>> int to_drain = min(count, pcp->batch << CONFIG_PCP_BATCH_SCALE_MAX);
>> count -= to_drain;
>
> It's wasteful to do a lock/unlock cycle just to read the count.
How about,
static void drain_pages_zone(unsigned int cpu, struct zone *zone)
{
struct per_cpu_pages *pcp = per_cpu_ptr(zone->per_cpu_pageset, cpu);
int count, to_drain;
do {
spin_lock(&pcp->lock);
to_drain = min(pcp->count, pcp->batch << CONFIG_PCP_BATCH_SCALE_MAX);
free_pcppages_bulk(zone, to_drain, pcp, 0);
spin_unlock(&pcp->lock);
} while (to_drain);
}
> It could
> rather look something like this:
>
Sorry, I don't follow your code...
> while (true)
> spin_lock(&pcp->lock);
> count = pcp->count;
> ...
> count -= to_drain;
> if (to_drain)
> drain_zone_pages(...)
Which subroutine does this code belong to, why it involves drain_zone_pages
> ...
> spin_unlock(&pcp->lock);
> if (count)
> break;
Thanks
Zhijian
