On Mon, Nov 1, 2021 at 1:37 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Fri 29-10-21 09:07:39, Suren Baghdasaryan wrote: > > On Fri, Oct 29, 2021 at 6:03 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > [...] > > > Well, I still do not see why that is a problem. This syscall is meant to > > > release the address space not to do it fast. > > > > It's the same problem for a userspace memory reaper as for the > > oom-reaper. The goal is to release the memory of the victim and to > > quickly move on to the next one if needed. > > The purpose of the oom_reaper is to _guarantee_ a forward progress. It > doesn't have to be quick or optimized for speed. Fair enough. Then the same guarantees should apply to userspace memory reapers. I think you clarified that well in your replies in https://lore.kernel.org/all/20170725154514.GN26723@xxxxxxxxxxxxxx: Because there is no _guarantee_ that the final __mmput will release the memory in finite time. And we cannot guarantee that longterm. ... __mmput calls into exit_aio and that can wait for completion and there is no way to guarantee this will finish in finite time. > > [...] > > > > Btw. the above code will not really tell you much on a larger machine > > > unless you manage to trigger mmap_sem contection. Otherwise you are > > > measuring the mmap_sem writelock fast path and that should be really > > > within a noise comparing to the whole address space destruction time. If > > > that is not the case then we have a real problem with the locking... > > > > My understanding of that discussion is that the concern was that even > > taking uncontended mmap_sem writelock would regress the exit path. > > That was what I wanted to confirm. Am I misreading it? > > No, your reading match my recollection. I just think that code > robustness in exchange of a rw semaphore write lock fast path is a > reasonable price to pay even if that has some effect on micro > benchmarks. 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? > -- > Michal Hocko > SUSE Labs
