On 2023-10-25 10:31, Steven Rostedt wrote:
On Wed, 25 Oct 2023 15:55:45 +0200 Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
[...] After digging lore for context, here are some thoughts about the actual proposal: AFAIU the intent here is to boost the scheduling slice for a userspace thread running with a mutex held so it can complete faster, and therefore reduce contention. I suspect this is not completely unrelated to priority inheritance futexes, except that one goal stated by Steven is to increase the owner slice without requiring to call a system call on the fast-path. Compared to PI futexes, I think Steven's proposal misses the part where a thread waiting on a futex boosts the lock owner's priority so it can complete faster. By making the lock owner selfishly claim that it needs a larger scheduling slice, it opens the door to scheduler disruption, and it's hard to come up with upper-bounds that work for all cases. Hopefully I'm not oversimplifying if I state that we have mainly two actors to consider: [A] the lock owner thread [B] threads that block trying to acquire the lock The fast-path here is [A]. [B] can go through a system call, I don't think it matters at all. So perhaps we can extend the rseq per-thread area with a field that implements a "held locks" list that allows [A] to let the kernel know that it is currently holding a set of locks (those can be chained when locks are nested). It would be updated on lock/unlock with just a few stores in userspace. Those lock addresses could then be used as keys for private locks, or transformed into inode/offset keys for shared-memory locks. Threads [B] blocking trying to acquire the lock can call a system call which would boost the lock owner's slice and/or priority for a given lock key. When the scheduler preempts [A], it would check whether the rseq per-thread area has a "held locks" field set and use this information to find the slice/priority boost which are currently active for each lock, and use this information to boost the task slice/priority accordingly. A scheme like this should allow lock priority inheritance without requiring system calls on the userspace lock/unlock fast path. Thoughts ? Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com
