Re: [PATCH v6 0/1] proc: Allow pid_revalidate() during LOOKUP_RCU

[Matthew Wilcox]

On Mon, Oct 04, 2021 at 10:56:28AM -0700, Stephen Brennan wrote:
> Problem Description:
> 
> When running running ~128 parallel instances of "TZ=/etc/localtime ps
> -fe >/dev/null" on a 128CPU machine, the %sys utilization reaches 97%,
> and perf shows the following code path as being responsible for heavy
> contention on the d_lockref spinlock:
> 
>       walk_component()
>         lookup_fast()
>           d_revalidate()
>             pid_revalidate() // returns -ECHILD
>           unlazy_child()
>             lockref_get_not_dead(&nd->path.dentry->d_lockref) <-- contention
> 
> The reason is that pid_revalidate() is triggering a drop from RCU to ref
> path walk mode. All concurrent path lookups thus try to grab a reference
> to the dentry for /proc/, before re-executing pid_revalidate() and then
> stepping into the /proc/$pid directory. Thus there is huge spinlock
> contention. This patch allows pid_revalidate() to execute in RCU mode,
> meaning that the path lookup can successfully enter the /proc/$pid
> directory while still in RCU mode. Later on, the path lookup may still
> drop into ref mode, but the contention will be much reduced at this
> point.
> 
> By applying this patch, %sys utilization falls to around 85% under the
> same workload, and the number of ps processes executed per unit time
> increases by 3x-4x. Although this particular workload is a bit
> contrived, we have seen some large collections of eager monitoring
> scripts which produced similarly high %sys time due to contention in the
> /proc directory.

I think it's perhaps also worth noting that this is a performance
regression relative to ... v5.4?  v4.14?  I forget the details; do you
have those to hand, Stephen?

(Yes, this is a stupid workload.  Yes, a customer really does have
this workload.)