Re: v6.2 client behaviour change (repeat access calls)?

["Linux regression tracking (Thorsten Leemhuis)" <regressions@leemhuis.info>]

[CCing the regression list, as it should be in the loop for regressions:
https://docs.kernel.org/admin-guide/reporting-regressions.html]

[also adding the author of the culprit (Trond) and the second NFS client
maintainer (Anna) to the list of recipients]

[TLDR: I'm adding this report to the list of tracked Linux kernel
regressions; the text you find below is based on a few templates
paragraphs you might have encountered already in similar form.
See link in footer if these mails annoy you.]

On 07.03.23 18:38, Daire Byrne wrote:
> I noticed a change in behaviour in the v6.2.x client versus v6.1.12 (and below).
> 
> We have some servers that mount Netapps from different locations many
> milliseconds away, and these contain apps and libs that get added to
> the LD_LIBRARY_PATH and PATH on remote login.
> 
> I then noticed that when I ssh'd into a remote server that had these
> mounts and the shell was starting, the first login was normal and I
> observed an expected flurry of lookups,getattrs and access calls for a
> grand total of only ~120 packets to the Netapp.
> 
> But when I disconnect and reconnect (ssh), now I see a flood of access
> calls to the netapp for a handful of repeating filehandles which look
> something like:
> 
>  2700 85.942563180 10.23.112.10 → 10.23.21.11  NFS 254 V3 ACCESS Call,
> FH: 0x7f36addc, [Check: RD LU MD XT DL]
>  2701 85.999838796  10.23.21.11 → 10.23.112.10 NFS 190 V3 ACCESS Reply
> (Call In 2700), [Allowed: RD LU MD XT DL]
>  2702 85.999970825 10.23.112.10 → 10.23.21.11  NFS 254 V3 ACCESS Call,
> FH: 0x7f36addc, [Check: RD LU MD XT DL]
>  2703 86.055340946  10.23.21.11 → 10.23.112.10 NFS 190 V3 ACCESS Reply
> (Call In 2702), [Allowed: RD LU MD XT DL]
>  2704 86.056865308 10.23.112.10 → 10.23.21.11  NFS 254 V3 ACCESS Call,
> FH: 0x7f36addc, [Check: RD LU MD XT DL]
>  2705 86.112233415  10.23.21.11 → 10.23.112.10 NFS 190 V3 ACCESS Reply
> (Call In 2704), [Allowed: RD LU MD XT DL]
> 
> This time we total 5000+ packets for this login which becomes very
> noticeable when the Netapp is 50ms away.
> 
> I didn't understand why the first login was fine but the second goes
> into this repeating access pattern. I set actimeo=3600 (long) but it
> does not seem to affect it.
> 
> I do not see this prior to v6.2 where repeated logins are equally fast
> and we don't see the repeating access calls.
> 
> So a bit of digging through the v6.2 changes and this looked like the
> relevant change:
> 
> commit 0eb43812c027 ("NFS: Clear the file access cache upon login”)
> [PATCH] NFS: Judge the file access cache's timestamp in rcu path?
> 
> I reverted those and got the prior (v6.1) performance.
> 
> What constitutes a login exactly? I also have services like "sysstat"
> or pcp that cause a systemd-logind to trigger regularly on our
> machines.... does that count and invalidate the cache?
> 
> Do the repeated access calls on the same handful of filehandles make
> sense? Even prior to those patches (or v6.1) there are only a couple
> of ACCESS calls to the Netapp on login.
> 
> We are a bit unique in that we run quite a few WAN high latency NFS
> workflows so are happy to trade long lived caches (e.g. actimeo and
> even nocto on occasion) for lower ops at the expense of total
> correctness.

Thanks for the report. To be sure the issue doesn't fall through the
cracks unnoticed, I'm adding it to regzbot, the Linux kernel regression
tracking bot:

#regzbot ^introduced 0eb43812c027
#regzbot title nfs: flood of access on second log-in (first is fine)
#regzbot ignore-activity

This isn't a regression? This issue or a fix for it are already
discussed somewhere else? It was fixed already? You want to clarify when
the regression started to happen? Or point out I got the title or
something else totally wrong? Then just reply and tell me -- ideally
while also telling regzbot about it, as explained by the page listed in
the footer of this mail.

Developers: When fixing the issue, remember to add 'Link:' tags pointing
to the report (the parent of this mail). See page linked in footer for
details.

Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)
--
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