Hello, On Fri, Oct 03, 2014 at 11:23:53AM -0700, Linus Torvalds wrote: > On Fri, Oct 3, 2014 at 10:07 AM, Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote: > > This teaches gup_fast and __gup_fast to re-enable irqs and > > cond_resched() if possible every BATCH_PAGES. > > This is disgusting. > > Many (most?) __gup_fast() users just want a single page, and the > stupid overhead of the multi-page version is already unnecessary. > This just makes things much worse. > > Quite frankly, we should make a single-page version of __gup_fast(), > and convert existign users to use that. After that, the few multi-page > users could have this extra latency control stuff. Ok. I didn't think at a better way to add the latency control other than to reduce nr_pages in a outer loop instead of altering the inner calls, but this is what I got after implementing it... If somebody has a cleaner way to implement the latency control stuff that's welcome and I'd be glad to replace it. > And yes, the single-page version of get_user_pages_fast() is actually > latency-critical. shared futexes hit it hard, and yes, I've seen this > in profiles. KVM would save a few cycles from a single-page version too. I just thought further optimizations could be added later and this was better than nothing. Considering I've no better idea how to implement the latency control stuff, for now I'll just drop this controversial patch, and I'll convert those get_user_pages to gup_unlocked instead of converting them to gup_fast, which is more than enough to obtain the mmap_sem holding scalability improvement (that also solves the mmap_sem trouble for the userfaultfd). gup_unlocked isn't as good as gup_fast but it's at least better than the current get_user_pages(). I got into this gup_fast latency control stuff purely because there were a few get_user_pages that could have been converted to get_user_pages_fast as they were using "current" and "current->mm" as the first two parameters, except for the risk of disabling irq for long. So I tried to do the right thing and fix gup_fast but I'll leave this further optimization queued for later. About the missing commit header for the other patch Paolo already replied to it, to clarify this a bit further in short I expect that FOLL_TRIED flag to be merged through the KVM git tree which already contains it. I'll add a comment to the commit header to specify it. Sorry for the confusion about that patch. Thanks, Andrea -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>