On Tue 03-06-25 22:22:46, Baolin Wang wrote: > Let me try to clarify further. > > The 'mm->rss_stat' is updated by using add_mm_counter(), > dec/inc_mm_counter(), which are all wrappers around > percpu_counter_add_batch(). In percpu_counter_add_batch(), there is percpu > batch caching to avoid 'fbc->lock' contention. OK, this is exactly the line of argument I was looking for. If _all_ updates done in the kernel are using batching and therefore the lock is only held every N (percpu_counter_batch) updates then a risk of locking contention would be decreased. This is worth having a note in the changelog. > This patch changes task_mem() > and task_statm() to get the accurate mm counters under the 'fbc->lock', but > this will not exacerbate kernel 'mm->rss_stat' lock contention due to the > the percpu batch caching of the mm counters. > > You might argue that my test cases cannot demonstrate an actual lock > contention, but they have already shown that there is no significant > 'fbc->lock' contention when the kernel updates 'mm->rss_stat'. I was arguing that `top -d 1' doesn't really represent a potential adverse usage. These proc files are generally readable so I would be expecting something like busy loop read while process tries to update counters to see the worst case scenario. If that is barely visible then we can conclude a normal use wouldn't even notice. See my point? -- Michal Hocko SUSE Labs
