On Tue, Jul 30, 2019 at 02:57:51PM +0200, Michal Hocko wrote: > [Cc Nick - the email thread starts http://lkml.kernel.org/r/20190729071037.241581-1-minchan@xxxxxxxxxx > A very brief summary is that mark_page_accessed seems to be quite > expensive and the question is whether we still need it and why > SetPageReferenced cannot be used instead. More below.] > > On Tue 30-07-19 21:39:35, Minchan Kim wrote: > > On Tue, Jul 30, 2019 at 02:32:37PM +0200, Michal Hocko wrote: > > > 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 > > > > I will include it. > > > > > 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. > > > > I couldn't come up with better solution. Feel free to suggest it. > > > > > > > > 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. > > > > commit bf3f3bc5e73 > > Author: Nick Piggin <npiggin@xxxxxxx> > > Date: Tue Jan 6 14:38:55 2009 -0800 > > > > mm: don't mark_page_accessed in fault path > > > > Doing a mark_page_accessed at fault-time, then doing SetPageReferenced at > > unmap-time if the pte is young has a number of problems. > > > > mark_page_accessed is supposed to be roughly the equivalent of a young pte > > for unmapped references. Unfortunately it doesn't come with any context: > > after being called, reclaim doesn't know who or why the page was touched. > > > > So calling mark_page_accessed not only adds extra lru or PG_referenced > > manipulations for pages that are already going to have pte_young ptes anyway, > > but it also adds these references which are difficult to work with from the > > context of vma specific references (eg. MADV_SEQUENTIAL pte_young may not > > wish to contribute to the page being referenced). > > > > Then, simply doing SetPageReferenced when zapping a pte and finding it is > > young, is not a really good solution either. SetPageReferenced does not > > correctly promote the page to the active list for example. So after removing > > mark_page_accessed from the fault path, several mmap()+touch+munmap() would > > have a very different result from several read(2) calls for example, which > > is not really desirable. > > Well, I have to say that this is rather vague to me. Nick, could you be > more specific about which workloads do benefit from this change? Let's > say that the zapped pte is the only referenced one and then reclaim > finds the page on inactive list. We would go and reclaim it. But does > that matter so much? Hot pages would be referenced from multiple ptes > very likely, no? As Nick mentioned in the description, without mark_page_accessed in zapping part, repeated mmap + touch + munmap never acticated the page while several read(2) calls easily promote it.
