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Received: from j130084.upc-j.chello.nl ([24.132.130.84] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1r0egF-0008Z5-JL; Wed, 08 Nov 2023 09:17:00 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id EA16630049D; Wed, 8 Nov 2023 10:16:59 +0100 (CET) Date: Wed, 8 Nov 2023 10:16:59 +0100 From: Peter Zijlstra To: Ankur Arora Cc: linux-kernel@vger.kernel.org, tglx@linutronix.de, torvalds@linux-foundation.org, paulmck@kernel.org, linux-mm@kvack.org, x86@kernel.org, akpm@linux-foundation.org, luto@kernel.org, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, willy@infradead.org, mgorman@suse.de, jon.grimm@amd.com, bharata@amd.com, raghavendra.kt@amd.com, boris.ostrovsky@oracle.com, konrad.wilk@oracle.com, jgross@suse.com, andrew.cooper3@citrix.com, mingo@kernel.org, bristot@kernel.org, mathieu.desnoyers@efficios.com, geert@linux-m68k.org, glaubitz@physik.fu-berlin.de, anton.ivanov@cambridgegreys.com, mattst88@gmail.com, krypton@ulrich-teichert.org, rostedt@goodmis.org, David.Laight@aculab.com, richard@nod.at, mjguzik@gmail.com Subject: Re: [RFC PATCH 40/86] context_tracking: add ct_state_cpu() Message-ID: <20231108091659.GK8262@noisy.programming.kicks-ass.net> References: <20231107215742.363031-1-ankur.a.arora@oracle.com> <20231107215742.363031-41-ankur.a.arora@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20231107215742.363031-41-ankur.a.arora@oracle.com> X-Rspam-User: X-Stat-Signature: xg6srpgqqh7pye6mfmkqcco5t85uaqwt X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 2B6CD140020 X-HE-Tag: 1699435039-212031 X-HE-Meta: U2FsdGVkX19hHqrD4Hgyxd9CDSUwxyaZbcwNP+ygWfkQOQKbVzum4QraVxnBv8ZSokgiTVCuPnjcQiALPRLRrGHf5dtXUvHGI96Wt5D+3L3hGMCpbtEfBVU4PAIAIAHbHHj4Q1MnYRyhS+UkTcKVQa62biftuVPh6V441Rbii9+tMCYZvtbRArOmIaGrVfRQbri/NlwtaAxii1h0RhNzuIqtcuxa/7pyIIprGY+zDphAATxLVWQ8jLFFOTz7GMupgDce42N2lfMIBIzEmLI6NQERO6fs3jMQY+cH12iapVKOLZYgvC5Tp0Y5fJu0YDi66ulJ1VrPgVgrs2Tx/2phGASlKsKWgGEweeNEIZioyFHLWUJeWW5a3Y2kbu2+/qtVCYTBEet7JBXbnqd7q8FkYRcIcWi1djTqLe5JXQHpTLY8id0Z3WiAqTIjAL2orDgRS9+LwYNC+O1wYk6/yDsOxugvKDDIaPJmFZZRVg/LbLWCYY120JVE6PrXkmKjdiEMqsTY1tCSByXJRv8GK6hTMDX5rJX3XqBiTRVusA/mn+7i3Il1g4l+oPQSNU5cdddTCaNDeqmRzwMWIzSfhA606sJ6CfhQf2CgPGkomBIH1WvwQx4MgHziEYabMxPgv2czCTOSmUSLmylTg14b4plz4sXgV3FV00HcXR6GCVYxuWhckBokAyGHLPUrCieN1MaDOlzvBwcFfrODJSZ6rXoGzWVtIhCXCcsUx99fsDaUV6WRHM+tJEORdSzQtdaen6uXLoGP2e4eyAMQrKaPqNbUJKYcG0NA0I9u1gB/2dBClb8f4vB7grq1P/CoAIgwKEAyJHxQWAa+OObknxiLxm1B71m0rK+XDtpK8stuYGvQloo7hakaDxOyTRunNnFviwyHWZx7xtw0O7QQFErauDC9o9tt6skjGnVj9d/Cwh0ew8nTIW4hjoxOQy0CDJUIvgaFU0hM1qmNe+SPLvOCUsT 3O3XVkO0 S3qdTxa27i6y8yP2Q8IkU601AlsYpi7GBH4qwY6JkIbHhATs1z4rOkXdRNRfLVHhxtcCcTOJqS9FrY8Ee8WjPY637niwNShf9988Im/RErJUTQTQqsxmYudlCX4lU29rCIRQTCtKWNV6PuL6KxQrJBXYpaCdngJrftp7JK8lUmt0zEPJBlSldmYNFwaa0hhTMzTR1QStXMuAXbT0= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Nov 07, 2023 at 01:57:26PM -0800, Ankur Arora wrote: > While making up its mind about whether to reschedule a target > runqueue eagerly or lazily, resched_curr() needs to know if the > target is executing in the kernel or in userspace. > > Add ct_state_cpu(). > > Signed-off-by: Ankur Arora > > --- > Using context-tracking for this seems like overkill. Is there a better > way to achieve this? One problem with depending on user_enter() is that > it happens much too late for our purposes. From the scheduler's > point-of-view the exit state has effectively transitioned once the > task exits the exit_to_user_loop() so we will see stale state > while the task is done with exit_to_user_loop() but has not yet > executed user_enter(). > > --- > include/linux/context_tracking_state.h | 21 +++++++++++++++++++++ > kernel/Kconfig.preempt | 1 + > 2 files changed, 22 insertions(+) > > diff --git a/include/linux/context_tracking_state.h b/include/linux/context_tracking_state.h > index bbff5f7f8803..6a8f1c7ba105 100644 > --- a/include/linux/context_tracking_state.h > +++ b/include/linux/context_tracking_state.h > @@ -53,6 +53,13 @@ static __always_inline int __ct_state(void) > { > return raw_atomic_read(this_cpu_ptr(&context_tracking.state)) & CT_STATE_MASK; > } > + > +static __always_inline int __ct_state_cpu(int cpu) > +{ > + struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu); > + > + return atomic_read(&ct->state) & CT_STATE_MASK; > +} > #endif > > #ifdef CONFIG_CONTEXT_TRACKING_IDLE > @@ -139,6 +146,20 @@ static __always_inline int ct_state(void) > return ret; > } > > +static __always_inline int ct_state_cpu(int cpu) > +{ > + int ret; > + > + if (!context_tracking_enabled_cpu(cpu)) > + return CONTEXT_DISABLED; > + > + preempt_disable(); > + ret = __ct_state_cpu(cpu); > + preempt_enable(); > + > + return ret; > +} Those preempt_disable/enable are pointless. But this patch is problematic, you do *NOT* want to rely on context tracking. Context tracking adds atomics to the entry path, this is slow and even with CONFIG_CONTEXT_TRACKING it is disabled until you configure the NOHZ_FULL nonsense. This simply cannot be.