Re: [PATCH 0/4 v2] mm/swap: Add locking for pagevec

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2019-04-24 05:15:52 [-0700], Matthew Wilcox wrote:
> On Wed, Apr 24, 2019 at 01:12:04PM +0200, Sebastian Andrzej Siewior wrote:
> > The swap code synchronizes its access to the (four) pagevec struct
> > (which is allocated per-CPU) by disabling preemption. This works and the
> > one struct needs to be accessed from interrupt context is protected by
> > disabling interrupts. This was manually audited and there is no lockdep
> > coverage for this.
> > There is one case where the per-CPU of a remote CPU needs to be accessed
> > and this is solved by started a worker on the remote CPU and waiting for
> > it to finish.
> > 
> > In v1 [0] it was attempted to add per-CPU spinlocks for the access to
> > struct. This would add lockdep coverage and access from a remote CPU so
> > the worker wouldn't be required.
> 
> >From my point of view, what is missing from this description is why we
> want to be able to access these structs from a remote CPU.  It's explained
> a little better in the 4/4 changelog, but I don't see any numbers that
> suggest what kinds of gains we might see (eg "reduces power consumption
> by x% on a particular setup", or even "average length of time in idle
> extended from x ms to y ms").

Pulling out a CPU from idle or userland computation looks bad. In the
first series I had numbers how long it takes to compute the loop for all
per-CPU data from one CPU vs the workqueue. Somehow the uncontended lock
was bad as per krobot report while I never got stable numbers from that
test.
The other motivation is RT where we need proper locking and can't use
that preempt-disable based locking.

Sebastian