Re: [PATCH] mm: swap: async free swap slot cache entries

From: David Rientjes <rientjes@google.com>
To: Chris Li <chrisl@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org,  linux-mm@kvack.org,
	Wei Xu <weixugc@google.com>,  Yu Zhao <yuzhao@google.com>,
	Greg Thelen <gthelen@google.com>,
	 Chun-Tse Shao <ctshao@google.com>,
	Suren Baghdasaryan <surenb@google.com>,
	 Yosry Ahmed <yosryahmed@google.com>,
	Brain Geffon <bgeffon@google.com>,
	 Minchan Kim <minchan@kernel.org>, Michal Hocko <mhocko@suse.com>,
	 Mel Gorman <mgorman@techsingularity.net>,
	 Huang Ying <ying.huang@intel.com>, Nhat Pham <nphamcs@gmail.com>,
	 Johannes Weiner <hannes@cmpxchg.org>,
	Kairui Song <kasong@tencent.com>,
	 Zhongkun He <hezhongkun.hzk@bytedance.com>,
	 Kemeng Shi <shikemeng@huaweicloud.com>,
	Barry Song <v-songbaohua@oppo.com>,
	 Hugh Dickins <hughd@google.com>
Subject: Re: [PATCH] mm: swap: async free swap slot cache entries
Date: Sun, 24 Dec 2023 13:13:35 -0800 (PST)	[thread overview]
Message-ID: <d171f8a4-47ed-0e29-877d-6824d593d7ed@google.com> (raw)
In-Reply-To: <CAF8kJuM59_12VNZ9d4oXZiLbAQC4LGLXODHqZw+7jiCJYva6YQ@mail.gmail.com>

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On Sun, 24 Dec 2023, Chris Li wrote:

> On Sat, Dec 23, 2023 at 7:01 PM David Rientjes <rientjes@google.com> wrote:
> >
> > On Sat, 23 Dec 2023, Chris Li wrote:
> >
> > > > How do you quantify the impact of the delayed swap_entry_free()?
> > > >
> > > > Since the free and memcg uncharge are now delayed, is there not the
> > > > possibility that we stay under memory pressure for longer?  (Assuming at
> > > > least some users are swapping because of memory pressure.)
> > > >
> > > > I would assume that since the free and uncharge itself is delayed that in
> > > > the pathological case we'd actually be swapping *more* until the async
> > > > worker can run.
> > >
> > > Thanks for raising this interesting question.
> > >
> > > First of all, the swap_entry_free() does not impact "memory.current".
> > > It reduces "memory.swap.current". Technically it is the swap pressure
> > > not memory pressure that suffers the extra delay.
> > >
> > > Secondly, we are talking about delaying up to 64 swap entries for a
> > > few microseconds.
> >
> > What guarantees that the async freeing happens within a few microseconds?
> 
> Linux kernel typically doesn't provide RT scheduling guarantees. You
> can change microseconds to milliseconds, my following reasoning still
> holds.
> 

What guarantees that the async freeing happens even within 10s?  Your 
responses are implying that there is some deadline by which this freeing 
absolutely must happen (us or ms), but I don't know of any strong 
guarantees.

If there are no absolute guarantees about when the freeing may now occur, 
I'm asking how the impact of the delayed swap_entry_free() can be 
quantified.

The benefit to the current implementation is that there *are* strong 
guarantees about when the freeing will occur and cannot grow exponentially 
before the async worker can do the freeing.