Re: [PATCH 16/23] sched: Use lightweight hazard pointers to grab lazy mms

[Rik van Riel <riel@surriel.com>]

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On Sun, 2022-01-09 at 17:52 -0700, Andy Lutomirski wrote:
> On Sun, Jan 9, 2022, at 1:51 PM, Linus Torvalds wrote:
> 
> 
> 
> > > exit_mmap();
> > > for each cpu still in mm_cpumask:
> > >   smp_load_acquire
> > > 
> > > That's it, unless the mm is actually in use
> > 
> > Ok, now do this for a machine with 1024 CPU's.
> > 
> > And tell me it is "scalable".
> > 
> 
> Do you mean a machine with 1024 CPUs and 2 bits set in mm_cpumask or
> 1024 CPU with 800 bits set in mm_cpumask?  In the former case, this
> is fine.  In the latter case, *on x86*, sure it does 800 loads, but
> we're about to do 800 CR3 writes to tear the whole mess down, so the
> 800 loads should be in the noise.  (And this series won't actually do
> this anyway on bare metal, since exit_mmap does the shootdown.)

Also, while 800 loads is kinda expensive, it is a heck of
a lot less expensive than 800 IPIs.

-- 
All Rights Reversed.

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