Date:   Tue, 8 Feb 2022 12:47:10 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Nicolas Saenz Julienne <nsaenzju@...hat.com>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, frederic@...nel.org, tglx@...utronix.de,
        mgorman@...e.de, linux-rt-users@...r.kernel.org, vbabka@...e.cz,
        cl@...ux.com, paulmck@...nel.org, willy@...radead.org
Subject: Re: [PATCH 2/2] mm/page_alloc: Add remote draining support to
 per-cpu lists

On Tue, Feb 08, 2022 at 11:07:50AM +0100, Nicolas Saenz Julienne wrote:
> The page allocator's per-cpu page lists (pcplists) are currently
> protected using local_locks. While performance savvy, this doesn't allow
> for remote access to these structures. CPUs requiring system-wide
> changes to the per-cpu lists get around this by scheduling
> workers on each CPU. That said, some setups like NOHZ_FULL CPUs,
> aren't well suited to this since they can't handle interruptions
> of any sort.
> 
> To mitigate this, replace the current draining mechanism with one that
> allows remotely draining the lists:
> 
>  - Each CPU now has two pcplists pointers: one that points to a pcplists
>    instance that is in-use, 'pcp->lp', another that points to an idle
>    and empty instance, 'pcp->drain'. CPUs access their local pcplists
>    through 'pcp->lp' and the pointer is dereferenced atomically.
> 
>  - When a CPU decides it needs to empty some remote pcplists, it'll
>    atomically exchange the remote CPU's 'pcp->lp' and 'pcp->drain'
>    pointers. A remote CPU racing with this will either have:
> 
>      - An old 'pcp->lp' reference, it'll soon be emptied by the drain
>        process, we just have to wait for it to finish using it.
> 
>      - The new 'pcp->lp' reference, that is, an empty pcplists instance.
>        rcu_replace_pointer()'s release semantics ensures any prior
>        changes will be visible by the remote CPU, for example: changes
>        to 'pcp->high' and 'pcp->batch' when disabling the pcplists.
> 
>  - The CPU that started the drain can now wait for an RCU grace period
>    to make sure the remote CPU is done using the old pcplists.
>    synchronize_rcu() counts as a full memory barrier, so any changes the
>    local CPU makes to the soon to be drained pcplists will be visible to
>    the draining CPU once it returns.
> 
>  - Then the CPU can safely free the old pcplists. Nobody else holds a
>    reference to it. Note that concurrent access to the remote pcplists
>    drain is protected by the 'pcpu_drain_mutex'.
> 
> >From an RCU perspective, we're only protecting access to the pcplists
> pointer, the drain operation is the writer and the local_lock critical
> sections are the readers. RCU guarantees atomicity both while
> dereferencing the pcplists pointer and replacing it. It also checks for
> RCU critical section/locking correctness, as all readers have to hold
> their per-cpu pagesets local_lock, which also counts as a critical
> section from RCU's perspective.
> 
> >From a performance perspective, on production configurations, the patch
> adds an extra dereference to all hot paths (under such circumstances
> rcu_dereference() will simplify to READ_ONCE()). Extensive measurements
> have been performed on different architectures to ascertain the
> performance impact is minimal. Most platforms don't see any difference
> and the worst-case scenario shows a 1-3% degradation on a page
> allocation micro-benchmark. See cover letter for in-depth results.
> 
> Accesses to the pcplists like the ones in mm/vmstat.c don't require RCU
> supervision since they can handle outdated data, but they do use
> rcu_access_pointer() to avoid compiler weirdness make sparse happy.
> 
> Note that special care has been taken to verify there are no races with
> the memory hotplug code paths. Notably with calls to zone_pcp_reset().
> As Mel Gorman explains in a previous patch[1]: "The existing hotplug
> paths guarantees the pcplists cannot be used after zone_pcp_enable()
> [the one in offline_pages()]. That should be the case already because
> all the pages have been freed and there is no page to put on the PCP
> lists."
> 
> All in all, this technique allows for remote draining on all setups with
> an acceptable performance impact. It benefits all sorts of use cases:
> low-latency, real-time, HPC, idle systems, KVM guests.
> 
> [1] 8ca559132a2d ("mm/memory_hotplug: remove broken locking of zone PCP
>     structures during hot remove")
> 
> Signed-off-by: Nicolas Saenz Julienne <nsaenzju@...hat.com>
> ---
> 
> Changes since RFC:
>  - Avoid unnecessary spin_lock_irqsave/restore() in free_pcppages_bulk()
>  - Add more detail to commit and code comments.
>  - Use synchronize_rcu() instead of synchronize_rcu_expedited(), the RCU
>    documentation says to avoid it unless really justified. I don't think
>    it's justified here, if we can schedule and join works, waiting for
>    an RCU grace period is OK.

https://patchwork.ozlabs.org/project/netdev/patch/1306228052.3026.16.camel@edumazet-laptop/

Adding 100ms to direct reclaim path might be problematic. It will also
slowdown kcompactd (note it'll call drain_all_pages for each zone).

>  - Avoid sparse warnings by using rcu_access_pointer() and
>    rcu_dereference_protected().
> 
>  include/linux/mmzone.h |  22 +++++-
>  mm/page_alloc.c        | 155 ++++++++++++++++++++++++++---------------
>  mm/vmstat.c            |   6 +-
>  3 files changed, 120 insertions(+), 63 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index b4cb85d9c6e8..b0b593fd8e48 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -388,13 +388,31 @@ struct per_cpu_pages {
>  	short expire;		/* When 0, remote pagesets are drained */
>  #endif
>  
> -	struct pcplists *lp;
> +	/*
> +	 * As a rule of thumb, any access to struct per_cpu_pages's 'lp' has
> +	 * happen with the pagesets local_lock held and using
> +	 * rcu_dereference_check(). If there is a need to modify both
> +	 * 'lp->count' and 'lp->lists' in the same critical section 'pcp->lp'
> +	 * can only be derefrenced once. See for example:

Typo.

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