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Tue, 03 Dec 2024 06:28:21 -0800 (PST) From: Changwoo Min X-Google-Original-From: Changwoo Min To: tj@kernel.org, void@manifault.com, mingo@redhat.com, peterz@infradead.org Cc: changwoo@igalia.com, kernel-dev@igalia.com, linux-kernel@vger.kernel.org Subject: [PATCH v3 2/5] sched_ext: Manage the validity of scx_rq_clock Date: Tue, 3 Dec 2024 23:27:59 +0900 Message-ID: <20241203142802.36305-3-changwoo@igalia.com> X-Mailer: git-send-email 2.47.1 In-Reply-To: <20241203142802.36305-1-changwoo@igalia.com> References: <20241203142802.36305-1-changwoo@igalia.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit An rq clock becomes valid when it is updated using update_rq_clock() and invalidated when the rq is unlocked using rq_unpin_lock(). Also, after long-running operations -- ops.running() and ops.update_idle() -- in a BPF scheduler, the sched_ext core invalidates the rq clock. Let's suppose the following timeline in the scheduler core: T1. rq_lock(rq) T2. update_rq_clock(rq) T3. a sched_ext BPF operation T4. rq_unlock(rq) T5. a sched_ext BPF operation T6. rq_lock(rq) T7. update_rq_clock(rq) For [T2, T4), we consider that rq clock is valid (SCX_RQ_CLK_VALID is set), so scx_bpf_clock_get_ns() calls during [T2, T4) (including T3) will return the rq clock updated at T2. For duration [T4, T7), when a BPF scheduler can still call scx_bpf_clock_get_ns() (T5), we consider the rq clock is invalid (SCX_RQ_CLK_VALID is unset at T4). So when calling scx_bpf_clock_get_ns() at T5, we will return a fresh clock value by calling sched_clock() internally. One example of calling scx_bpf_clock_get_ns(), when the rq clock is invalid (like T5), is in scx_central [1]. The scx_central scheduler uses a BPF timer for preemptive scheduling. In every msec, the timer callback checks if the currently running tasks exceed their timeslice. At the beginning of the BPF timer callback (central_timerfn in scx_central.bpf.c), scx_central gets the current time. When the BPF timer callback runs, the rq clock could be invalid, the same as T5. In this case, scx_bpf_clock_get_ns() returns a fresh clock value rather than returning the old one (T2). [1] https://github.com/sched-ext/scx/blob/main/scheds/c/scx_central.bpf.c Signed-off-by: Changwoo Min --- kernel/sched/core.c | 6 +++++- kernel/sched/ext.c | 3 +++ kernel/sched/sched.h | 2 +- 3 files changed, 9 insertions(+), 2 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 95e40895a519..ab8015c8cab4 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -789,6 +789,7 @@ static void update_rq_clock_task(struct rq *rq, s64 delta) void update_rq_clock(struct rq *rq) { s64 delta; + u64 clock; lockdep_assert_rq_held(rq); @@ -800,11 +801,14 @@ void update_rq_clock(struct rq *rq) SCHED_WARN_ON(rq->clock_update_flags & RQCF_UPDATED); rq->clock_update_flags |= RQCF_UPDATED; #endif + clock = sched_clock_cpu(cpu_of(rq)); + scx_rq_clock_update(rq, clock); - delta = sched_clock_cpu(cpu_of(rq)) - rq->clock; + delta = clock - rq->clock; if (delta < 0) return; rq->clock += delta; + update_rq_clock_task(rq, delta); } diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index 7fff1d045477..ac279a657d50 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -2928,6 +2928,8 @@ static void set_next_task_scx(struct rq *rq, struct task_struct *p, bool first) if (SCX_HAS_OP(running) && (p->scx.flags & SCX_TASK_QUEUED)) SCX_CALL_OP_TASK(SCX_KF_REST, running, p); + scx_rq_clock_stale(rq); + clr_task_runnable(p, true); /* @@ -3590,6 +3592,7 @@ void __scx_update_idle(struct rq *rq, bool idle) { int cpu = cpu_of(rq); + scx_rq_clock_stale(rq); if (SCX_HAS_OP(update_idle) && !scx_rq_bypassing(rq)) { SCX_CALL_OP(SCX_KF_REST, update_idle, cpu_of(rq), idle); if (!static_branch_unlikely(&scx_builtin_idle_enabled)) diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index da55b39c9e58..7d26731c350f 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -1779,7 +1779,7 @@ static inline void rq_unpin_lock(struct rq *rq, struct rq_flags *rf) if (rq->clock_update_flags > RQCF_ACT_SKIP) rf->clock_update_flags = RQCF_UPDATED; #endif - + scx_rq_clock_stale(rq); lockdep_unpin_lock(__rq_lockp(rq), rf->cookie); } -- 2.47.1