On 05.04.23 13:41, Peter Zijlstra wrote:
On Wed, Apr 05, 2023 at 01:10:07PM +0200, Frederic Weisbecker wrote:
On Wed, Apr 05, 2023 at 12:44:04PM +0200, Frederic Weisbecker wrote:
On Tue, Apr 04, 2023 at 04:42:24PM +0300, Yair Podemsky wrote:
+ int state = atomic_read(&ct->state);
+ /* will return true only for cpus in kernel space */
+ return state & CT_STATE_MASK == CONTEXT_KERNEL;
+}
Also note that this doesn't stricly prevent userspace from being interrupted.
You may well observe the CPU in kernel but it may receive the IPI later after
switching to userspace.
We could arrange for avoiding that with marking ct->state with a pending work bit
to flush upon user entry/exit but that's a bit more overhead so I first need to
know about your expectations here, ie: can you tolerate such an occasional
interruption or not?
Bah, actually what can we do to prevent from that racy IPI? Not much I fear...
Yeah, so I don't think that's actually a problem. The premise is that
*IFF* NOHZ_FULL stays in userspace, then it will never observe the IPI.
If it violates this by doing syscalls or other kernel entries; it gets
to keep the pieces.
Yair is currently on vacation, so I'm replying on his behalf.
Indeed, RT userspace is supposed to not call into the kernel, that's the
premise.
--
Thanks,
David / dhildenb
