From: David Hildenbrand <david@redhat.com>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: Dave Hansen <dave.hansen@intel.com>,
Andrew Morton <akpm@linux-foundation.org>,
Hillf Danton <hdanton@sina.com>,
Dave Hansen <dave.hansen@linux.intel.com>,
Vlastimil Babka <vbabka@suse.cz>,
Michal Hocko <mhocko@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Linux-MM <linux-mm@kvack.org>, "Tang, Feng" <feng.tang@intel.com>
Subject: Re: [PATCH 0/6 v2] Calculate pcp->high based on zone sizes and active CPUs
Date: Fri, 28 May 2021 12:21:20 +0200 [thread overview]
Message-ID: <8d6dfad6-ae0b-fd47-1d83-23baf82921b4@redhat.com> (raw)
In-Reply-To: <20210528100918.GM30378@techsingularity.net>
On 28.05.21 12:09, Mel Gorman wrote:
> On Fri, May 28, 2021 at 11:52:53AM +0200, David Hildenbrand wrote:
>>>> "Disable pcplists so that page isolation cannot race with freeing
>>>> in a way that pages from isolated pageblock are left on pcplists."
>>>>
>>>> Guess we'd then want to move the draining before start_isolate_page_range()
>>>> in alloc_contig_range().
>>>>
>>>
>>> Or instead of draining, validate the PFN range in alloc_contig_range
>>> is within the same zone and if so, call zone_pcp_disable() before
>>> start_isolate_page_range and enable after __alloc_contig_migrate_range.
>>>
>>
>> We require the caller to only pass a range within a single zone, so that
>> should be fine.
>>
>> The only ugly thing about zone_pcp_disable() is
>> mutex_lock(&pcp_batch_high_lock) which would serialize all
>> alloc_contig_range() and even with offline_pages().
>>
>
> True so it would have to be accessed if that is bad or not. If racing
> against offline_pages, memory is potentially being offlined in the
> target zone which may cause allocation failure. If racing with other
> alloc_contig_range calls, the two callers are potentially racing to
> isolate and allocate the same range. The arguement could be made that
> alloc_contig_ranges should be serialised within one zone to improve the
> allocation success rate at the potential cost of allocation latency.
We have 3 main users of alloc_contig_range():
1. CMA
CMA synchronizes allocation within a CMA area via the allocation bitmap.
So parallel CMA is perfectly possible and avoids races by design.
2. alloc_contig_pages() / Gigantic pages
Gigantic page allocation could race with virtio-mem. CMA does not apply.
Possible but unlikely to matter in practice. virtio-mem will retry later
again.
3. virito-mem
A virtio-mem device only operates on its assigned memory region, so we
cannot have alloc_contig_range() from different devices racing, even
when within a single zone. It could only race with gigantic pages as CMA
does not apply.
So serializing would mostly harm parallel CMA (possible and likely) and
parallel virtio-mem operation (e.g., unplug memory of two virtio-mem
devices -- unlikely but possible).
Memory offlining racing with CMA is not an issue (impossible).
virtio-mem synchronizes with memory offlining via memory notifiers,
there is only a tiny little race window that usually doesn't matter as
virtio-mem is expected to usually triggers offlining itself, and not
user space rancomly. Memory offlining can race with dynamic gigantic
page allocation, wich is highly unreliable already.
I wonder if we could optimize locking in zone_pcp_disable() instead.
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2021-05-28 10:21 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-25 8:01 [PATCH 0/6 v2] Calculate pcp->high based on zone sizes and active CPUs Mel Gorman
2021-05-25 8:01 ` [PATCH 1/6] mm/page_alloc: Delete vm.percpu_pagelist_fraction Mel Gorman
2021-05-26 17:41 ` Vlastimil Babka
2021-05-25 8:01 ` [PATCH 2/6] mm/page_alloc: Disassociate the pcp->high from pcp->batch Mel Gorman
2021-05-26 18:14 ` Vlastimil Babka
2021-05-27 10:52 ` Mel Gorman
2021-05-28 10:27 ` Vlastimil Babka
2021-05-25 8:01 ` [PATCH 3/6] mm/page_alloc: Adjust pcp->high after CPU hotplug events Mel Gorman
2021-05-28 11:08 ` Vlastimil Babka
2021-05-25 8:01 ` [PATCH 4/6] mm/page_alloc: Scale the number of pages that are batch freed Mel Gorman
2021-05-28 11:19 ` Vlastimil Babka
2021-05-25 8:01 ` [PATCH 5/6] mm/page_alloc: Limit the number of pages on PCP lists when reclaim is active Mel Gorman
2021-05-28 11:43 ` Vlastimil Babka
2021-05-25 8:01 ` [PATCH 6/6] mm/page_alloc: Introduce vm.percpu_pagelist_high_fraction Mel Gorman
2021-05-28 11:59 ` Vlastimil Babka
2021-05-28 12:53 ` Mel Gorman
2021-05-28 14:38 ` Vlastimil Babka
2021-05-27 19:36 ` [PATCH 0/6 v2] Calculate pcp->high based on zone sizes and active CPUs Dave Hansen
2021-05-28 8:55 ` Mel Gorman
2021-05-28 9:03 ` David Hildenbrand
2021-05-28 9:08 ` David Hildenbrand
2021-05-28 9:49 ` Mel Gorman
2021-05-28 9:52 ` David Hildenbrand
2021-05-28 10:09 ` Mel Gorman
2021-05-28 10:21 ` David Hildenbrand [this message]
2021-05-28 12:12 ` Vlastimil Babka
2021-05-28 12:37 ` Mel Gorman
2021-05-28 14:39 ` Dave Hansen
2021-05-28 15:18 ` Mel Gorman
2021-05-28 16:17 ` Dave Hansen
2021-05-31 12:00 ` Feng Tang
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