Hello, On Wed, Apr 12, 2023 at 02:40:53PM -0400, Waiman Long wrote: > On 4/12/23 14:27, Tejun Heo wrote: > > On Tue, Apr 11, 2023 at 09:36:01AM -0400, Waiman Long wrote: > > > The newly introduced cpuset_can_fork() and cpuset_cancel_fork() calls > > > are only needed when the CLONE_INTO_CGROUP flag is set which is not > > > likely. Adding an extra cpuset_can_fork() call does introduce a bit > > > of performance overhead in the fork/clone fastpath. To reduce this > > > performance overhead, introduce a new clone_into_cgroup_can_fork flag > > > into the cgroup_subsys structure. This flag, when set, will call the > > > can_fork and cancel_fork methods only if the CLONE_INTO_CGROUP flag > > > is set. > > > > > > The cpuset code is now modified to set this flag. The same cpuset > > > checking code in cpuset_can_fork() and cpuset_cancel_fork() will have > > > to stay as the cgroups can be different, but the cpusets may still be > > > the same. So the same check must be present in both cpuset_fork() and > > > cpuset_can_fork() to make sure that attach_in_progress is correctly set. > > > > > > Signed-off-by: Waiman Long <longman@xxxxxxxxxx> > > Waiman, I'm not necessarily against this optimization but can we at least > > have some performance numbers to show that this is actually meaningful? > > Given how heavy our fork path is, I'm not too sure this would show up in any > > meaningful way. > > That make sense to me. I am OK to leave it for now as it is an optimization > patch anyway. > > BTW, another question that I have is about the cgroup_threadgroup_rwsem. It > is currently a percpu rwsem. Is it possible to change it into a regular > rwsem instead? It is causing quite a bit of latency for workloads that > require rather frequent changes to cgroups. I know we have a "favordynmods" > mount option to disable the percpu operation. This will still be less > performant than a normal rwsem. Of course the downside is that the fork/exit > fastpaths will be slowed down a bit. I don't know. Maybe? A rwsem actually has a scalability factor in that the more CPUs are forking, the more expensive the rwsem becomes, so it is a bit more of a concern. Another factor is that in majority of use cases we're almost completely bypassing write-locking percpu_rwsem, so it feel a bit sad to convert it to a regular rwsem. So, if favordynmods is good enough, I'd like to keep it that way. Thanks. -- tejun
