Date:   Thu, 28 Apr 2022 12:58:51 -0700
From:   Minchan Kim <minchan@...nel.org>
To:     Johannes Weiner <hannes@...xchg.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 Thu, Apr 28, 2022 at 02:34:28PM -0400, Johannes Weiner wrote:
> On Thu, Apr 28, 2022 at 10:31:45AM -0700, Minchan Kim wrote:
> > On Thu, Apr 28, 2022 at 01:23:21PM -0400, Johannes Weiner wrote:
> > > On Thu, Apr 28, 2022 at 09:59:53AM -0700, Minchan Kim wrote:
> > > > On Thu, Apr 28, 2022 at 10:25:59AM -0400, Johannes Weiner wrote:
> > > > > On Wed, Apr 27, 2022 at 03:16:48PM -0700, Minchan Kim wrote:
> > > > > > On Wed, Apr 27, 2022 at 05:20:29PM -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.
> > > > > > 
> > > > > > Thanks! Please consider ZSWPIN to rename more general term, too.
> > > > > 
> > > > > That doesn't make sense to me.
> > > > > 
> > > > > Zram is a swap backend, its traffic is accounted in PSWPIN/OUT. Zswap
> > > > > is a writeback cache on top of the swap backend. It has pages
> > > > > entering, refaulting, and being written back to the swap backend
> > > > > (PSWPOUT). A zswpout and a zramout are different things.
> > > > 
> > > > Think about that system has two swap devices (storage + zram).
> > > > I think it's useful to know how many swap IO comes from zram
> > > > and rest of them are storage.
> > > 
> > > Hm, isn't this comparable to having one swap on flash and one swap on
> > > a rotating disk? /sys/block/*/stat should be able to tell you how
> > > traffic is distributed, no?
> > 
> > That raises me a same question. Could you also look at the zswap stat
> > instead of adding it into vmstat? (If zswap doesn't have the counter,
> > couldn't we simply add new stat in sysfs?)
> 
> My point is that for regular swap backends there is already
> PSWP*. Distinguishing traffic between two swap backends is legitimate
> of course, but zram is not really special compared to other backends
> from that POV. It's only special in its memory consumption.
> 
> zswap *is* special, though. Even though some people use it *like* a
> swap backend, it's also a cache on top of swap. zswap loads and stores
> do not show up in PSWP*. And they shouldn't, because in a cache
> configuration, you still need the separate PSWP* stats to understand
> cache eviction behavior and cache miss ratio. memory -> zswap is
> ZSWPOUT; zswap -> disk is PSWPOUT; PSWPIN is a cache miss etc.
> 
> > I thought the patch aims for exposting statistics to grab easier
> > using popular meminfo and vmstat and wanted to leverage it for
> > zram, too.
> 
> Right. zram and zswap overlap in their functionality and have similar
> deficits in their stats. Both should be fixed, I'm not opposing
> that. But IMO we should be careful about conflating
> them. Fundamentally, one is a block device, the other is an MM-native
> cache layer that sits on top of block devices. Drawing false
> equivalencies between them will come back to haunt us.

Make sense to me.

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