Re: [PATCH V2 2/2] Documentation: filesystems: introduce proc/slabinfo to users

[zhang fangzheng <fangzheng.zhang1003@gmail.com>]

On Tue, Feb 20, 2024 at 5:21 PM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 2/20/24 09:49, zhang fangzheng wrote:
> > On Mon, Feb 19, 2024 at 4:09 PM Vlastimil Babka <vbabka@suse.cz> wrote:
> >>
> >> On 2/19/24 07:23, zhang fangzheng wrote:
> >> > On Mon, Feb 19, 2024 at 12:24 PM Matthew Wilcox <willy@infradead.org> wrote:
> >> >>
> >> >> On Mon, Feb 19, 2024 at 11:19:11AM +0800, Fangzheng Zhang wrote:
> >> >> > +Note, <slabreclaim> comes from the collected results in the file
> >> >> > +/sys/kernel/slab/$cache/reclaim_account. Next, we will mark /proc/slabinfo
> >> >> > +as deprecated and recommend the use of either sysfs directly or
> >> >> > +use of the "slabinfo" tool that we have been providing in linux/tools/mm.
> >> >>
> >> >> Wait, so you're going to all of the trouble of changing the format of
> >> >> slabinfo (with the associated costs of updating every tool that currently
> >> >> parses it), only to recommend that we stop using it and start using
> >> >> tools/mm/slabinfo instead?
> >> >>
> >>
> >> Hi,
> >>
> >> > The initial purpose was to obtain the type of each slab through
> >> > a simple command 'cat proc/slabinfo'. So here, my intention is not to
> >> > update all slabinfo-related tools for the time being, but to modify
> >> > the version number of proc/slabinfo and further display the results
> >> > of using the command.
> >>
> >> I'm not sure you understand the concern. There are existing consumers of
> >> /proc/slabinfo, that might become broken by patch 1/2. We don't even know
> >> them all, they might not be all opensource etc. So we can't even make sure
> >> all of them are updated. What can happen after patch 1/2:
> >> - they keep working and ignore the new column (good)
> >> - they include a version check and notice a new unsupported version and
> >> refuse to work
> >> - confused by the new column they start throwing error, or report wrong
> >> stats (that's worse)
> >>
> > I generally understand your concerns about modifying patch 1/2.
> >
> > But judging from my modifications, this worry does not seem to be valid.
> > Because the “/proc/slabinfo” is not used in related slabinfo debugging tools
> > (such as tools/mm/slabinfo),
>
> Hi,
>
> we are not concerned about slabinfo debugging tools that are part of kernel
> source tree, but about those outside, including those created privately and
> we don't even know they exist.
>
For your concerns, I think the supplementary introduction that new
output results
of slabinfo v2.2  in patch 2/2 will be necessary. This can help them
optimize their tools
more quickly to adapt to proc/slabinfo. Is this more friendly?

> > but "/sys/kernel/slab/<slab_name>/" (in
> > Documentation/mm/slub.rst) or "/ sys/kernel/debug/slab" (in
> > tools/mm/slabinfo.c).
> >
> > Furthermore, the current modification only involves optimizing the output
> > of proc/slabinfo,
>
> It's not "only", the output of /proc/slabinfo is what those tools consume,
> so that's what concerns us the most.
>
> > and does not modify the  struct slabinfo or struct kmem_cache.
> > So there is no need to adapt other modifications.
>
> These on the other hand are internal details of the kernel which we can
> modify as much we want
>
> >> >> How about we simply do nothing?
> >>
> >> Agreed wrt modifying /proc/slabinfo
> >>
> >> > The note here means what changes will occur after
> >> > we modify the version number of proc/slabinfo to 2.2.
> >> > As for the replacement of tools/mm/slabinfo (that inspired
> >> > by Christoph’s suggestions), it will be implemented in the next version
> >> > or even the later version.
> >>
> >> So what is your motivation for all this in the first place? You have some
> >> monitoring tool that relies on /proc/slabinfo and want to distinguish
> >> reclaimable caches? So you can change it to parse the /sys directories. Is
> >> it more work? Yes, but you only have to do that once per boot, because
> >> unlike the object/memory stats in /proc/slabinfo, the reclaimable flag will
> >> not change for a cache.
> >>
> > The situation as you mentioned is very suitable for my current needs.
> > My original intention is just to get an intuitive slab screen through a
> > simple ‘cat proc/slabinfo’ command. As for the description "<slabreclaim>
>
> That would be nice, but again we must be careful about existing consumers of
> /proc/slabinfo so we can't always have nice things.
>
> > comes from the collected results in the file
> > /sys/kernel/slab/$cache/reclaim_account"
> > may not be appropriate. Here I want to express that the column <slabreclaim> has
> > the same effect as traversing "/sys/kernel/slab/$ cache/reclaim_account".
> >
> >> Would tools/mm/slabinfo almost work for you, but you're missing something?
> >> Then send patches for that in the first place. Changing /proc/slabinfo (and
> >> breaking other consumers) for a quick and easy fix with a different solution
> >> planned for the future is simply not feasible.
> >>
> > Using the slabinfo tool to parse /sys/kernel/slab/$cache/reclaim_account
> > is what I think about optimizing future tools during the discussion.
> > It will not affect the current patch 1/2, and patch 2/2 is mainly to
> > supplement the output examples of proc/slabinfo.
> >
> > If the community is willing to accept it, I will only modify
> > patch 1/2 to implement it.
> >
> > Thanks very much!
> >
> >> HTH,
> >> Vlastimil
> >>
> >> > Thanks!
> >>
>