linux-kernel - Re: [PATCH] Enable haltpoll for arm64

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Date:   Tue, 29 Aug 2023 17:31:27 +0300
From:   Mihai Carabas <mihai.carabas@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Petr Mladek <pmladek@...e.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Juerg Haefliger <juerg.haefliger@...onical.com>,
        Mickaël Salaün <mic@...ikod.net>,
        Joao Martins <joao.m.martins@...cle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Ankur Arora <ankur.a.arora@...cle.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kvm@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH] Enable haltpoll for arm64

> Using poll_state as is on arm64 seems sub-optimal, would not something
> like the below make sense?
>
> ---
> diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
> index 9b6d90a72601..9ab40198b042 100644
> --- a/drivers/cpuidle/poll_state.c
> +++ b/drivers/cpuidle/poll_state.c
> @@ -27,7 +27,11 @@ static int __cpuidle poll_idle(struct cpuidle_device *dev,
>   		limit = cpuidle_poll_time(drv, dev);
>   
>   		while (!need_resched()) {
> -			cpu_relax();
> +
> +			smp_cond_load_relaxed(current_thread_info()->flags,
> +					      (VAL & TIF_NEED_RESCHED) ||
> +					      (loop_count++ >= POLL_IDLE_RELAX_COUNT));
> +
>   			if (loop_count++ < POLL_IDLE_RELAX_COUNT)
>   				continue;
>   

Thank you for the suggestion. I have tried it and also different 
variations like [1] to respect the initial logic
but I obtain poor performance compared to the initial one:

perf bench sched pipe
# Running 'sched/pipe' benchmark:
# Executed 1000000 pipe operations between two processes

      Total time: 136.215 [sec]

      136.215229 usecs/op
            7341 ops/sec


[1]

--- a/drivers/cpuidle/poll_state.c
+++ b/drivers/cpuidle/poll_state.c
@@ -26,12 +26,16 @@ static int __cpuidle poll_idle(struct cpuidle_device 
*dev,

                 limit = cpuidle_poll_time(drv, dev);

-               while (!need_resched()) {
-                       cpu_relax();
-                       if (loop_count++ < POLL_IDLE_RELAX_COUNT)
-                               continue;
-
+               for (;;) {
                         loop_count = 0;
+
+ smp_cond_load_relaxed(&current_thread_info()->flags,
+                                             (VAL & TIF_NEED_RESCHED) ||
+                                             (loop_count++ >= 
POLL_IDLE_RELAX_COUNT));
+
+                       if (loop_count < POLL_IDLE_RELAX_COUNT)
+                               break;
+
                         if (local_clock_noinstr() - time_start > limit) {
                                 dev->poll_time_limit = true;
                                 break;