On 20/11/24 18:30, Frederic Weisbecker wrote:
Le Wed, Nov 20, 2024 at 06:10:43PM +0100, Valentin Schneider a écrit :
On 20/11/24 15:23, Frederic Weisbecker wrote:
Ah but there is CT_STATE_GUEST and I see the last patch also applies that to CT_STATE_IDLE.
So that could be:
bool ct_set_cpu_work(unsigned int cpu, unsigned int work) { struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu); unsigned int old; bool ret = false;
preempt_disable();
old = atomic_read(&ct->state);
/* CT_STATE_IDLE can be added to last patch here */ if (!(old & (CT_STATE_USER | CT_STATE_GUEST))) { old &= ~CT_STATE_MASK; old |= CT_STATE_USER; }
Hmph, so that lets us leverage the cmpxchg for a !CT_STATE_KERNEL check, but we get an extra loop if the target CPU exits kernelspace not to userspace (e.g. vcpu or idle) in the meantime - not great, not terrible.
The thing is, what you read with atomic_read() should be close to reality. If it already is != CT_STATE_KERNEL then you're good (minus racy changes). If it is CT_STATE_KERNEL then you still must do a failing cmpxchg() in any case, at least to make sure you didn't miss a context tracking change. So the best you can do is a bet.
At the cost of one extra bit for the CT_STATE area, with CT_STATE_KERNEL=1 we could do:
old = atomic_read(&ct->state); old &= ~CT_STATE_KERNEL;
And perhaps also old |= CT_STATE_IDLE (I'm seeing the last patch now), so you at least get a chance of making it right (only ~CT_STATE_KERNEL will always fail) and CPUs usually spend most of their time idle.
I'm thinking with:
CT_STATE_IDLE = 0, CT_STATE_USER = 1, CT_STATE_GUEST = 2, CT_STATE_KERNEL = 4, /* Keep that as a standalone bit */
we can stick with old &= ~CT_STATE_KERNEL; and that'll let the cmpxchg succeed for any of IDLE/USER/GUEST.
Thanks.