linux-kernel - Re: [PATCH 4/5] mm: zswap: add basic meminfo and vmstat coverage

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Date:   Thu, 28 Apr 2022 10:05:13 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, Roman Gushchin <guro@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Seth Jennings <sjenning@...hat.com>,
        Dan Streetman <ddstreet@...e.org>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 4/5] mm: zswap: add basic meminfo and vmstat coverage

On Wed, Apr 27, 2022 at 03:12:17PM -0700, Minchan Kim wrote:
> On Wed, Apr 27, 2022 at 05:36:26PM -0400, Johannes Weiner wrote:
> > On Wed, Apr 27, 2022 at 05:20:31PM -0400, Johannes Weiner wrote:
> > > On Wed, Apr 27, 2022 at 01:29:34PM -0700, Minchan Kim wrote:
> > > > Hi Johannes,
> > > > 
> > > > On Wed, Apr 27, 2022 at 12:00:15PM -0400, Johannes Weiner wrote:
> > > > > Currently it requires poking at debugfs to figure out the size and
> > > > > population of the zswap cache on a host. There are no counters for
> > > > > reads and writes against the cache. As a result, it's difficult to
> > > > > understand zswap behavior on production systems.
> > > > > 
> > > > > Print zswap memory consumption and how many pages are zswapped out in
> > > > > /proc/meminfo. Count zswapouts and zswapins in /proc/vmstat.
> > > > > 
> > > > > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> > > > > ---
> > > > >  fs/proc/meminfo.c             |  7 +++++++
> > > > >  include/linux/swap.h          |  5 +++++
> > > > >  include/linux/vm_event_item.h |  4 ++++
> > > > >  mm/vmstat.c                   |  4 ++++
> > > > >  mm/zswap.c                    | 13 ++++++-------
> > > > >  5 files changed, 26 insertions(+), 7 deletions(-)
> > > > > 
> > > > > diff --git a/fs/proc/meminfo.c b/fs/proc/meminfo.c
> > > > > index 6fa761c9cc78..6e89f0e2fd20 100644
> > > > > --- a/fs/proc/meminfo.c
> > > > > +++ b/fs/proc/meminfo.c
> > > > > @@ -86,6 +86,13 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
> > > > >  
> > > > >  	show_val_kb(m, "SwapTotal:      ", i.totalswap);
> > > > >  	show_val_kb(m, "SwapFree:       ", i.freeswap);
> > > > > +#ifdef CONFIG_ZSWAP
> > > > > +	seq_printf(m,  "Zswap:          %8lu kB\n",
> > > > > +		   (unsigned long)(zswap_pool_total_size >> 10));
> > > > > +	seq_printf(m,  "Zswapped:       %8lu kB\n",
> > > > > +		   (unsigned long)atomic_read(&zswap_stored_pages) <<
> > > > > +		   (PAGE_SHIFT - 10));
> > > > > +#endif
> > > > 
> > > > I agree it would be very handy to have the memory consumption in meminfo
> > > > 
> > > > https://lore.kernel.org/all/YYwZXrL3Fu8%2FvLZw@google.com/
> > > > 
> > > > If we really go this Zswap only metric instead of general term
> > > > "Compressed", I'd like to post maybe "Zram:" with same reason
> > > > in this patchset. Do you think that's better idea instead of
> > > > introducing general term like "Compressed:" or something else?
> > > 
> > > I'm fine with changing it to Compressed. If somebody cares about a
> > > more detailed breakdown, we can add Zswap, Zram subsets as needed.
> > 
> > It does raise the question what to do about cgroup, though. Should the
> > control files (memory.zswap.current & memory.zswap.max) apply to zram
> > in the future? If so, we should rename them, too.
> > 
> > I'm not too familiar with zram, maybe you can provide some
> > background. AFAIU, Google uses zram quite widely; all the more
> > confusing why there is no container support for it yet.
> 
> My usecase with zram is Android which doesn't use memcg.

Ok.

After more thought, my take is that in the future it could make sense
to track zram pages in a cgroup's memory.current. But it should NOT be
included in the dedicated memory.zswap.* files. Zswap is an in-kernel
writeback cache, and those files allow userspace to tune writeback
thresholds depending on the composition of the workload's
workingset. This doesn't translate to zram: the wb facility that it
has is triggered by hand, based on criteria such as idle pages and
compression rate. It's not based on size. From a cgroup POV, it's a
memory consumer that should be subject to memory.max, nothing more.

This distinction applies to meminfo as well, though. While I think it
makes sense to have a combined "Compressed" counter for zram and
zswap, it's still important to understand zswap behavior on its own to
tune the system-wide writeback threshold in max_pool_percent. (And
again, while zram can also be limited, it's not a writeback threshold,
it's just a red line for returning -ENOMEM).

So I'm going to keep the Zswap and Zswapped items and retract the
delta patch for renaming it to Compressed.

But I'd ack a patch that adds a combined "Compressed" counter for zram
+ zswap if you send it, Minchan.