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Wysocki" Cc: "Rafael J. Wysocki" , Linux PM , Aboorva Devarajan , LKML , Daniel Lezcano , Artem Bityutskiy References: <4953183.GXAFRqVoOG@rjwysocki.net> <13679187.uLZWGnKmhe@rjwysocki.net> <842b1500-9f4f-47e5-9777-ee89351f956e@arm.com> Content-Language: en-US From: Christian Loehle In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 1/10/25 13:34, Rafael J. Wysocki wrote: > On Fri, Jan 10, 2025 at 2:16 PM Christian Loehle > wrote: >> >> On 1/10/25 12:53, Rafael J. Wysocki wrote: >>> From: Rafael J. Wysocki >>> >>> After commit 6da8f9ba5a87 ("cpuidle: teo: Skip tick_nohz_get_sleep_length() >>> call in some cases") the teo governor behaves a bit differently on >>> systems where idle state 0 is a "polling" state (that is, it is not >>> really an idle state, but a loop continuously executed by the CPU). >>> Namely, on such systems it skips the tick_nohz_get_sleep_length() call >>> if the target residency of the current candidate idle state is small >>> enough. >>> >>> However, if state 0 itself was to be returned, it would be returned >>> right away without calling tick_nohz_get_sleep_length() even on systems >>> where it was not a "polling" state until commit 4b20b07ce72f ("cpuidle: >>> teo: Don't count non-existent intercepts") that attempted to fix this >>> problem. >>> >>> Unfortunately, commit 4b20b07ce72f has made the governor always call >>> tick_nohz_get_sleep_length() when about to return state 0 early, even >>> if that state is a "polling" one, which is inconsistent and defeats >>> the purpose of commit 6da8f9ba5a87 in that case. >>> >>> Address this by adding a CPUIDLE_FLAG_POLLING check to the path where >>> state 0 is returned early to prevent tick_nohz_get_sleep_length() from >>> being called if it is a "polling" state. >>> >>> Fixes: 4b20b07ce72f ("cpuidle: teo: Don't count non-existent intercepts") >>> Signed-off-by: Rafael J. Wysocki >>> --- >>> drivers/cpuidle/governors/teo.c | 3 ++- >>> 1 file changed, 2 insertions(+), 1 deletion(-) >>> >>> --- a/drivers/cpuidle/governors/teo.c >>> +++ b/drivers/cpuidle/governors/teo.c >>> @@ -422,7 +422,8 @@ >>> first_suitable_idx = i; >>> } >>> } >>> - if (!idx && prev_intercept_idx) { >>> + if (!idx && prev_intercept_idx && >>> + !(drv->states[0].flags & CPUIDLE_FLAG_POLLING)) { >>> /* >>> * We have to query the sleep length here otherwise we don't >>> * know after wakeup if our guess was correct. >>> >>> >>> >> >> But then you do run into the issue of intercepts not being detected if >> state0 is the right choice, don't you? > > That's true, but then on systems with a "polling" state 0 you still > have this problem if the state returned early is not state 0. Say C1 > on x86.> > The point here is that the behavior needs to be consistent, one way or another. Yes, gotcha. Why not be consistent 'in the other way' then? > > The exact point of commit 6da8f9ba5a87 was to avoid calling > tick_nohz_get_sleep_length() in some cases when the state to be > returned is shallow enough and obviously that includes a "polling" > state 0, possibly at the cost of being somewhat inaccurate in > prediction. Somewhat inaccurate meaning not making any prediction? cpu_data->sleep_length_ns = KTIME_MAX; How much is the harm for calling tick_nohz_get_sleep_length() when polling anyway? I know tick_nohz_get_sleep_length() is the majority of the usual cpuidle entry path, but for many scenarios where state0 is appropriate that should be pretty fast, no? > > Then you're seeing this intercept accumulation for state 0 when there > are only 2 states in the table (or all of the other states are much > higher target residency than state 0). > > Commit 4b20b07ce72f effectively caused tick_nohz_get_sleep_length() to > be called every time on systems without a "polling" state 0, which was > fair enough, but it also affected the other systems, which wasn't. > >> This would then enable intercept-detection only for <50% of the time, >> another option is to not allow intercepts selecting a polling state, but >> there were recent complaints about this exact behavior from Aboorva (+TO). >> They don't have a low-latency non-polling state. >> >> https://lore.kernel.org/lkml/20240809073120.250974-1-aboorvad@linux.ibm.com/ > > If they don't have a "polling" state 0, they won't be affected by this > patch and after commit 4b20b07ce72f, they'll always call > tick_nohz_get_sleep_length(), so the current governor behavior is > generally unsuitable for them. They do though. commit 5ddcc03a07ae ("powerpc/cpuidle: Set CPUIDLE_FLAG_POLLING for snooze state") So they have a polling 'snooze' and a relatively high latency (hundreds usecs) non-polling state and no deeper state. So if they don't query sleep length on snooze on a (1us)-interrupt-wakeup heavy workload they will get 50% state0 and 50% state1 (because intercepts recovered due to not querying sleep length). > > I have an idea how to change it to be more accurate in prediction, but > we'll see how it goes. Stay tuned. I will. Regards, Christian