On Wed, Sep 22, 2021 at 09:33:43PM +0200, Peter Zijlstra wrote:
Anyway, lemme see if I get your proposal; lets say the counter starts at
0 and is in kernel space.
0x00(0) - kernel
0x02(2) - user
0x04(0) - kernel
So far so simple, then NMI on top of that goes:
0x00(0) - kernel
0x03(3) - kernel + nmi
0x04(0) - kernel
0x06(2) - user
0x09(1) - user + nmi
0x0a(2) - user
Which then gives us:
(0) := kernel
(1) := nmi-from-user
(2) := user
(3) := nmi-from-kernel
Which should work I suppose. But like I said above, I'd be happier if
this counter would live in context_tracking rather than RCU.
Furthermore, if we have this counter, the we can also do things like:
seq = context_tracking_seq_cpu(that_cpu);
if ((seq & 3) != USER)
// nohz_fail, do something
set_tsk_thread_flag(curr_task(that_cpu), TIF_DO_SOME_WORK);
if (seq == context_tracking_seq_cpu(that_cpu))
// success!!
To remotely set pending state. Allowing yet more NOHZ_FULL fixes, like,
for example, eliding the text_poke IPIs.