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SubjectRe: [External] Re: [PATCH v2] mm: add new syscall pidfd_set_mempolicy().
On Thu 17-11-22 15:19:20, Zhongkun He wrote:
> Hi Michal, thanks for your replay.
>
> >
> > It would be better to add the patch that has been tested.
>
> OK.
>
> >
> > One way to deal with that would be to use a similar model as css_tryget
>
> Percpu_ref is a good way to reduce memory footprint in fast path.But it
> has the potential to make mempolicy heavy. the sizeof mempolicy is 32
> bytes and it may not have a long life time, which duplicated from the
> parent in fork().If we modify atomic_t to percpu_ref, the efficiency of
> reading in fastpath will increase, the efficiency of creation and
> deletion will decrease, and the occupied space will increase
> significantly.I am not really sure it is worth it.
>
> atomic_t; 4
> sizeof(percpu_ref + percpu_ref_data + cpus* unsigned long)
> 16+56+cpus*8

Yes the memory consumption is going to increase but the question is
whether this is something that is a real problem. Is it really common to
have many vmas with a dedicated policy?

What I am arguing here is that there are essentially 2 ways forward.
Either we continue to build up on top of the existing and arguably very
fragile code and make it even more subtle or follow a general pattern of
a proper reference counting (with usual tricks to reduce cache line
bouncing and similar issues). I do not really see why memory policies
should be any different and require very special treatment.

> > Btw. have you tried to profile those slowdowns to identify hotspots?
> >
> > Thanks
>
> Yes, it will degrade performance about 2%-%3 may because of the task_lock
> and atomic operations on the reference count as shown
> in the previous email.
>
> new hotspots in perf.
> 1.34% [kernel] [k] __mpol_put
> 0.53% [kernel] [k] _raw_spin_lock
> 0.44% [kernel] [k] get_task_policy

Thanks!
--
Michal Hocko
SUSE Labs

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