RE: Upper bound mode for kernel timers

[David Laight]

From: Josh Poimboeuf
> Sent: 02 March 2021 00:11
> 
> We had a report of a regression in the TCP keepalive timer.  The user
> had a 3600s keepalive timer for preventing firewall disconnects (on a
> 3650s interval).  They observed keepalive timers coming in up to four
> minutes late, causing unexpected disconnects.
> 
> The regression was observed to have come from the timer wheel rewrite
> from almost five years ago:
> 
>   500462a9de65 ("timers: Switch to a non-cascading wheel")
> 
> As you mentioned, with a HZ of 1000, the granularity for a one-hour
> timer is four minutes, which matches the seen behavior.

That seems horribly broken - if technically valid.

Reading the big comment even the 32sec for the next finer 'wheel'
seems a little coarse for a 1h timer.
The second finer wheel has 4sec resolution - which is probably reasonable.

	David

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