On Sat, Apr 23, 2022 at 02:43:13PM -0700, Yosry Ahmed wrote: [...] > > > + cg_run_nowait(memcg, alloc_pagecache_50M_noexit, (void *)(long)fd); > > > + sleep(1); > > > > These sleep(1)s do not seem robust. Since kernel keeps the page cache > > around, you can convert anon to use tmpfs and use simple cg_run to > > trigger the allocations of anon (tmpfs) and file which will remain in > > memory even after return from cg_run. > > Other tests in the file are also using sleep approach (see > test_memcg_min, although it retries for multiple times until > memory.current reaches an expected amount). In my experience it hasn't > been flaky running for multiple times on different machines, but I > agree it can be flaky (false negative). > If other tests are doing the same then ignore this comment for now. There should be a separate effort to move towards more deterministic approach for the tests instead of sleep(). > I am not sure about the allocating file pages with cg_run, is it > guaranteed that the page cache will remain in memory until the test > ends? If it doesn't, it can also flake, but it would produce false > positives (the test could pass because the kernel drained page cache > for some other reason although the interface is not working > correctly). > > In my personal opinion, false negative flakes are better than false > positives. At least currently the test explicitly and clearly fails if > the allocations are not successful. If we rely on the page cache > remaining until the test finishes then it could silently pass if the > interface is not working correctly. > > There are a few ways we can go forward with this: > 1) Keep everything as-is, but print a message if the test fails due to > memory.current not reaching 100MB to make it clear that it didn't fail > due to a problem with the interface. > 2) Add a sleep/retry loop similar to test_memcg_min instead of sleeping once. > 3) Send a signal from forked children when they are done with the > allocation, and wait to receive this signal in the test to make sure > the allocation is completed. > > In my opinion we should do (1) (and maybe (2)) for now as (3) could be > an overkill if the test is normal passing. Maybe add a comment about > (3) being an option in the future if the test flakes. Let me know what > you think? I am ok with (1).
