On Tue, Nov 2, 2021 at 8:14 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > On Tue, Nov 2, 2021 at 12:58 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > On Mon 01-11-21 08:44:58, Suren Baghdasaryan wrote: > > [...] > > > I'm with you on this one, that's why I wanted to measure the price we > > > would pay. Below are the test results: > > > > > > Test: https://lore.kernel.org/all/20170725142626.GJ26723@xxxxxxxxxxxxxx/ > > > Compiled: gcc -O2 -static test.c -o test > > > Test machine: 128 core / 256 thread 2x AMD EPYC 7B12 64-Core Processor > > > (family 17h) > > > > > > baseline (Linus master, f31531e55495ca3746fb895ffdf73586be8259fa) > > > p50 (median) 87412 > > > p95 168210 > > > p99 190058 > > > average 97843.8 > > > stdev 29.85% > > > > > > unconditional mmap_write_lock in exit_mmap (last column is the change > > > from the baseline) > > > p50 (median) 88312 +1.03% > > > p95 170797 +1.54% > > > p99 191813 +0.92% > > > average 97659.5 -0.19% > > > stdev 32.41% > > > > > > unconditional mmap_write_lock in exit_mmap + Matthew's patch (last > > > column is the change from the baseline) > > > p50 (median) 88807 +1.60% > > > p95 167783 -0.25% > > > p99 187853 -1.16% > > > average 97491.4 -0.36% > > > stdev 30.61% > > > > > > stdev is quite high in all cases, so the test is very noisy. > > > The impact seems quite low IMHO. WDYT? > > > > Results being very noisy is what I recall as well. Thanks! > > I believe, despite the noise, the percentiles show that overall we do > not noticeably regress the exit path by taking mmap_lock > unconditionally. > If there are no objections, I would like to post a patchset which > implements unconditional locking in exit_mmap() and process_madvise() > calling __oom_reap_task_mm() under protection of read mmap_lock. > Thanks! Discussing how the patch I want to post works for maple trees that Matthew is working on, I've got a question: IIUC, according to Michal's post here: https://lore.kernel.org/all/20170725154514.GN26723@xxxxxxxxxxxxxx, unmap_vmas() can race with other mmap_lock read holders (including oom_reap_task_mm()) with no issues. Maple tree patchset requires rcu read lock or the mmap semaphore be held (read or write side) when walking the tree, including inside unmap_vmas(). When asked, he told me that he is not sure why it's currently "safe" to walk the vma->vm_next list in unmap_vmas() while another thread is reaping the mm. Michal (or maybe someone else), could you please clarify why unmap_vmas() can safely race with oom_reap_task_mm()? Or maybe my understanding was wrong? Thanks, Suren. > > > -- > > Michal Hocko > > SUSE Labs
