Re: [PATCH] mm: release the spinlock on zap_pte_range

[Michal Hocko <mhocko@kernel.org>]

On Tue 30-07-19 21:11:10, Minchan Kim wrote:
> On Mon, Jul 29, 2019 at 10:35:15AM +0200, Michal Hocko wrote:
> > On Mon 29-07-19 17:20:52, Minchan Kim wrote:
> > > On Mon, Jul 29, 2019 at 09:45:23AM +0200, Michal Hocko wrote:
> > > > On Mon 29-07-19 16:10:37, Minchan Kim wrote:
> > > > > In our testing(carmera recording), Miguel and Wei found unmap_page_range
> > > > > takes above 6ms with preemption disabled easily. When I see that, the
> > > > > reason is it holds page table spinlock during entire 512 page operation
> > > > > in a PMD. 6.2ms is never trivial for user experince if RT task couldn't
> > > > > run in the time because it could make frame drop or glitch audio problem.
> > > > 
> > > > Where is the time spent during the tear down? 512 pages doesn't sound
> > > > like a lot to tear down. Is it the TLB flushing?
> > > 
> > > Miguel confirmed there is no such big latency without mark_page_accessed
> > > in zap_pte_range so I guess it's the contention of LRU lock as well as
> > > heavy activate_page overhead which is not trivial, either.
> > 
> > Please give us more details ideally with some numbers.
> 
> I had a time to benchmark it via adding some trace_printk hooks between
> pte_offset_map_lock and pte_unmap_unlock in zap_pte_range. The testing
> device is 2018 premium mobile device.
> 
> I can get 2ms delay rather easily to release 2M(ie, 512 pages) when the
> task runs on little core even though it doesn't have any IPI and LRU
> lock contention. It's already too heavy.
> 
> If I remove activate_page, 35-40% overhead of zap_pte_range is gone
> so most of overhead(about 0.7ms) comes from activate_page via
> mark_page_accessed. Thus, if there are LRU contention, that 0.7ms could
> accumulate up to several ms.

Thanks for this information. This is something that should be a part of
the changelog. I am sorry to still poke into this because I still do not
have a full understanding of what is going on and while I do not object
to drop the spinlock I still suspect this is papering over a deeper
problem.

If mark_page_accessed is really expensive then why do we even bother to
do it in the tear down path in the first place? Why don't we simply set
a referenced bit on the page to reflect the young pte bit? I might be
missing something here of course.

-- 
Michal Hocko
SUSE Labs