Andrew Morton wrote: > On Tue, 22 Sep 2015 17:42:13 -0700 Greg Thelen <gthelen@xxxxxxxxxx> wrote: > >> Andrew Morton wrote: >> >> > On Tue, 22 Sep 2015 15:16:32 -0700 Greg Thelen <gthelen@xxxxxxxxxx> wrote: >> > >> >> mem_cgroup_read_stat() returns a page count by summing per cpu page >> >> counters. The summing is racy wrt. updates, so a transient negative sum >> >> is possible. Callers don't want negative values: >> >> - mem_cgroup_wb_stats() doesn't want negative nr_dirty or nr_writeback. >> >> - oom reports and memory.stat shouldn't show confusing negative usage. >> >> - tree_usage() already avoids negatives. >> >> >> >> Avoid returning negative page counts from mem_cgroup_read_stat() and >> >> convert it to unsigned. >> > >> > Someone please remind me why this code doesn't use the existing >> > percpu_counter library which solved this problem years ago. >> > >> >> for_each_possible_cpu(cpu) >> > >> > and which doesn't iterate across offlined CPUs. >> >> I found [1] and [2] discussing memory layout differences between: >> a) existing memcg hand rolled per cpu arrays of counters >> vs >> b) array of generic percpu_counter >> The current approach was claimed to have lower memory overhead and >> better cache behavior. >> >> I assume it's pretty straightforward to create generic >> percpu_counter_array routines which memcg could use. Possibly something >> like this could be made general enough could be created to satisfy >> vmstat, but less clear. >> >> [1] http://www.spinics.net/lists/cgroups/msg06216.html >> [2] https://lkml.org/lkml/2014/9/11/1057 > > That all sounds rather bogus to me. __percpu_counter_add() doesn't > modify struct percpu_counter at all except for when the cpu-local > counter overflows the configured batch size. And for the memcg > application I suspect we can set the batch size to INT_MAX... Nod. The memory usage will be a bit larger, but the code reuse is attractive. I dusted off Vladimir's https://lkml.org/lkml/2014/9/11/710. Next step is to benchmark it before posting. -- 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>