On Tue, 15 Oct 2024, Ankur Arora wrote: > > Alternatively, if we get an IPI anyway, we can avoid smp_cond_load() and > > rely on need_resched() and some new delay/cpu_relax() API that waits for > > a timeout or an IPI, whichever comes first. E.g. cpu_relax_timeout() > > which on arm64 it's just a simplified version of __delay() without the > > 'while' loops. > > AFAICT when polling (which we are since poll_idle() calls > current_set_polling_and_test()), the scheduler will elide the IPI > by remotely setting the need-resched bit via set_nr_if_polling(). The scheduler runs on multiple cores. The core on which we are running this code puts the core into a wait state so the scheduler does not run on this core at all during the wait period. The other cores may run scheduler functions and set the need_resched bit for the core where we are currently waiting. The other core will wake our core up by sending an IPI. The IPI will invoke a scheduler function on our core and the WFE will continue. > Once we stop polling then the scheduler should take the IPI path > because call_function_single_prep_ipi() will fail. The IPI stops the polling. IPI is an interrupt.
