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From: Andrea Righi To: Tejun Heo Cc: David Vernet , Changwoo Min , Dan Schatzberg , Emil Tsalapatis , sched-ext@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: [PATCH 05/13] sched_ext: Simplify breather mechanism with scx_aborting flag Message-ID: References: <20251109183112.2412147-1-tj@kernel.org> <20251109183112.2412147-6-tj@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20251109183112.2412147-6-tj@kernel.org> X-Migadu-Flow: FLOW_OUT On Sun, Nov 09, 2025 at 08:31:04AM -1000, Tejun Heo wrote: > The breather mechanism was introduced in 62dcbab8b0ef ("sched_ext: Avoid > live-locking bypass mode switching") and e32c260195e6 ("sched_ext: Enable the > ops breather and eject BPF scheduler on softlockup") to prevent live-locks by > injecting delays when CPUs are trapped in dispatch paths. > > Currently, it uses scx_breather_depth (atomic_t) and scx_in_softlockup > (unsigned long) with separate increment/decrement and cleanup operations. The > breather is only activated when aborting, so tie it directly to the exit > mechanism. Replace both variables with scx_aborting flag set when exit is > claimed and cleared after bypass is enabled. Introduce scx_claim_exit() to > consolidate exit_kind claiming and breather enablement. This eliminates > scx_clear_softlockup() and simplifies scx_softlockup() and scx_bypass(). > > The breather mechanism will be replaced by a different abort mechanism in a > future patch. This simplification prepares for that change. Acked-by: Andrea Righi Thanks, -Andrea > > Cc: Dan Schatzberg > Cc: Emil Tsalapatis > Signed-off-by: Tejun Heo > --- > kernel/sched/ext.c | 54 +++++++++++++++++++++------------------------- > 1 file changed, 25 insertions(+), 29 deletions(-) > > diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c > index 4b8b91494947..905d01f74687 100644 > --- a/kernel/sched/ext.c > +++ b/kernel/sched/ext.c > @@ -33,9 +33,8 @@ static DEFINE_MUTEX(scx_enable_mutex); > DEFINE_STATIC_KEY_FALSE(__scx_enabled); > DEFINE_STATIC_PERCPU_RWSEM(scx_fork_rwsem); > static atomic_t scx_enable_state_var = ATOMIC_INIT(SCX_DISABLED); > -static unsigned long scx_in_softlockup; > -static atomic_t scx_breather_depth = ATOMIC_INIT(0); > static int scx_bypass_depth; > +static bool scx_aborting; > static bool scx_init_task_enabled; > static bool scx_switching_all; > DEFINE_STATIC_KEY_FALSE(__scx_switched_all); > @@ -1834,7 +1833,7 @@ static void scx_breather(struct rq *rq) > > lockdep_assert_rq_held(rq); > > - if (likely(!atomic_read(&scx_breather_depth))) > + if (likely(!READ_ONCE(scx_aborting))) > return; > > raw_spin_rq_unlock(rq); > @@ -1843,9 +1842,9 @@ static void scx_breather(struct rq *rq) > > do { > int cnt = 1024; > - while (atomic_read(&scx_breather_depth) && --cnt) > + while (READ_ONCE(scx_aborting) && --cnt) > cpu_relax(); > - } while (atomic_read(&scx_breather_depth) && > + } while (READ_ONCE(scx_aborting) && > time_before64(ktime_get_ns(), until)); > > raw_spin_rq_lock(rq); > @@ -3740,30 +3739,14 @@ void scx_softlockup(u32 dur_s) > goto out_unlock; > } > > - /* allow only one instance, cleared at the end of scx_bypass() */ > - if (test_and_set_bit(0, &scx_in_softlockup)) > - goto out_unlock; > - > printk_deferred(KERN_ERR "sched_ext: Soft lockup - CPU%d stuck for %us, disabling \"%s\"\n", > smp_processor_id(), dur_s, scx_root->ops.name); > > - /* > - * Some CPUs may be trapped in the dispatch paths. Enable breather > - * immediately; otherwise, we might even be able to get to scx_bypass(). > - */ > - atomic_inc(&scx_breather_depth); > - > scx_error(sch, "soft lockup - CPU#%d stuck for %us", smp_processor_id(), dur_s); > out_unlock: > rcu_read_unlock(); > } > > -static void scx_clear_softlockup(void) > -{ > - if (test_and_clear_bit(0, &scx_in_softlockup)) > - atomic_dec(&scx_breather_depth); > -} > - > /** > * scx_bypass - [Un]bypass scx_ops and guarantee forward progress > * @bypass: true for bypass, false for unbypass > @@ -3826,8 +3809,6 @@ static void scx_bypass(bool bypass) > ktime_get_ns() - bypass_timestamp); > } > > - atomic_inc(&scx_breather_depth); > - > /* > * No task property is changing. We just need to make sure all currently > * queued tasks are re-queued according to the new scx_rq_bypassing() > @@ -3883,10 +3864,8 @@ static void scx_bypass(bool bypass) > raw_spin_rq_unlock(rq); > } > > - atomic_dec(&scx_breather_depth); > unlock: > raw_spin_unlock_irqrestore(&bypass_lock, flags); > - scx_clear_softlockup(); > } > > static void free_exit_info(struct scx_exit_info *ei) > @@ -3981,6 +3960,7 @@ static void scx_disable_workfn(struct kthread_work *work) > > /* guarantee forward progress by bypassing scx_ops */ > scx_bypass(true); > + WRITE_ONCE(scx_aborting, false); > > switch (scx_set_enable_state(SCX_DISABLING)) { > case SCX_DISABLING: > @@ -4103,9 +4083,24 @@ static void scx_disable_workfn(struct kthread_work *work) > scx_bypass(false); > } > > -static void scx_disable(enum scx_exit_kind kind) > +static bool scx_claim_exit(struct scx_sched *sch, enum scx_exit_kind kind) > { > int none = SCX_EXIT_NONE; > + > + if (!atomic_try_cmpxchg(&sch->exit_kind, &none, kind)) > + return false; > + > + /* > + * Some CPUs may be trapped in the dispatch paths. Enable breather > + * immediately; otherwise, we might not even be able to get to > + * scx_bypass(). > + */ > + WRITE_ONCE(scx_aborting, true); > + return true; > +} > + > +static void scx_disable(enum scx_exit_kind kind) > +{ > struct scx_sched *sch; > > if (WARN_ON_ONCE(kind == SCX_EXIT_NONE || kind == SCX_EXIT_DONE)) > @@ -4114,7 +4109,7 @@ static void scx_disable(enum scx_exit_kind kind) > rcu_read_lock(); > sch = rcu_dereference(scx_root); > if (sch) { > - atomic_try_cmpxchg(&sch->exit_kind, &none, kind); > + scx_claim_exit(sch, kind); > kthread_queue_work(sch->helper, &sch->disable_work); > } > rcu_read_unlock(); > @@ -4435,9 +4430,8 @@ static void scx_vexit(struct scx_sched *sch, > const char *fmt, va_list args) > { > struct scx_exit_info *ei = sch->exit_info; > - int none = SCX_EXIT_NONE; > > - if (!atomic_try_cmpxchg(&sch->exit_kind, &none, kind)) > + if (!scx_claim_exit(sch, kind)) > return; > > ei->exit_code = exit_code; > @@ -4653,6 +4647,8 @@ static int scx_enable(struct sched_ext_ops *ops, struct bpf_link *link) > */ > WARN_ON_ONCE(scx_set_enable_state(SCX_ENABLING) != SCX_DISABLED); > WARN_ON_ONCE(scx_root); > + if (WARN_ON_ONCE(READ_ONCE(scx_aborting))) > + WRITE_ONCE(scx_aborting, false); > > atomic_long_set(&scx_nr_rejected, 0); > > -- > 2.51.1 >