From f7d1b585e1533e26801c13569b96b84b2ad2d3c1 Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Tue, 5 Nov 2024 11:45:24 -1000 Subject: [PATCH 1/3] sched_ext: Add a missing newline at the end of an error message Signed-off-by: Tejun Heo --- kernel/sched/ext.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index 74344a43ccf1..3bdb08fc2056 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -4974,7 +4974,7 @@ static int scx_ops_enable(struct sched_ext_ops *ops, struct bpf_link *link) if (!cpumask_equal(housekeeping_cpumask(HK_TYPE_DOMAIN), cpu_possible_mask)) { - pr_err("sched_ext: Not compatible with \"isolcpus=\" domain isolation"); + pr_err("sched_ext: Not compatible with \"isolcpus=\" domain isolation\n"); return -EINVAL; } From a759bf0dfc4db3cb6556fc79c7c98da3a46b2b80 Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Tue, 5 Nov 2024 11:45:27 -1000 Subject: [PATCH 2/3] sched_ext: Update scx_show_state.py to match scx_ops_bypass_depth's new type 0e7ffff1b811 ("scx: Fix raciness in scx_ops_bypass()") converted scx_ops_bypass_depth from an atomic to an int. Update scx_show_state.py accordingly. Signed-off-by: Tejun Heo Fixes: 0e7ffff1b811 ("scx: Fix raciness in scx_ops_bypass()") --- tools/sched_ext/scx_show_state.py | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/tools/sched_ext/scx_show_state.py b/tools/sched_ext/scx_show_state.py index 8bc626ede1c4..c4b3fdda9a0b 100644 --- a/tools/sched_ext/scx_show_state.py +++ b/tools/sched_ext/scx_show_state.py @@ -35,6 +35,6 @@ print(f'enabled : {read_static_key("__scx_ops_enabled")}') print(f'switching_all : {read_int("scx_switching_all")}') print(f'switched_all : {read_static_key("__scx_switched_all")}') print(f'enable_state : {ops_state_str(enable_state)} ({enable_state})') -print(f'bypass_depth : {read_atomic("scx_ops_bypass_depth")}') +print(f'bypass_depth : {prog["scx_ops_bypass_depth"].value_()}') print(f'nr_rejected : {read_atomic("scx_nr_rejected")}') print(f'enable_seq : {read_atomic("scx_enable_seq")}') From a6250aa251eacaf3ebfcfe152a96a727fd483ecd Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Sat, 9 Nov 2024 10:43:55 -1000 Subject: [PATCH 3/3] sched_ext: Handle cases where pick_task_scx() is called without preceding balance_scx() sched_ext dispatches tasks from the BPF scheduler from balance_scx() and thus every pick_task_scx() call must be preceded by balance_scx(). While this usually holds, due to a bug, there are cases where the fair class's balance() returns true indicating that it has tasks to run on the CPU and thus terminating balance() calls but fails to actually find the next task to run when pick_task() is called. In such cases, pick_task_scx() can be called without preceding balance_scx(). Detect this condition using SCX_RQ_BAL_PENDING flags. If detected, keep running the previous task if possible and avoid stalling from entering idle without balancing. Signed-off-by: Tejun Heo Cc: Peter Zijlstra Link: http://lkml.kernel.org/r/Ztj_h5c2LYsdXYbA@slm.duckdns.org --- kernel/sched/core.c | 13 ++++++++----- kernel/sched/ext.c | 44 +++++++++++++++++++++++++++++++------------- kernel/sched/sched.h | 5 +++-- 3 files changed, 42 insertions(+), 20 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index aeb595514461..a910a5b4c274 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -5914,12 +5914,15 @@ static void prev_balance(struct rq *rq, struct task_struct *prev, #ifdef CONFIG_SCHED_CLASS_EXT /* - * SCX requires a balance() call before every pick_next_task() including - * when waking up from SCHED_IDLE. If @start_class is below SCX, start - * from SCX instead. + * SCX requires a balance() call before every pick_task() including when + * waking up from SCHED_IDLE. If @start_class is below SCX, start from + * SCX instead. Also, set a flag to detect missing balance() call. */ - if (scx_enabled() && sched_class_above(&ext_sched_class, start_class)) - start_class = &ext_sched_class; + if (scx_enabled()) { + rq->scx.flags |= SCX_RQ_BAL_PENDING; + if (sched_class_above(&ext_sched_class, start_class)) + start_class = &ext_sched_class; + } #endif /* diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index 3bdb08fc2056..19f9cb3a4190 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -2634,7 +2634,7 @@ static int balance_one(struct rq *rq, struct task_struct *prev) lockdep_assert_rq_held(rq); rq->scx.flags |= SCX_RQ_IN_BALANCE; - rq->scx.flags &= ~SCX_RQ_BAL_KEEP; + rq->scx.flags &= ~(SCX_RQ_BAL_PENDING | SCX_RQ_BAL_KEEP); if (static_branch_unlikely(&scx_ops_cpu_preempt) && unlikely(rq->scx.cpu_released)) { @@ -2948,12 +2948,11 @@ static struct task_struct *pick_task_scx(struct rq *rq) { struct task_struct *prev = rq->curr; struct task_struct *p; + bool prev_on_scx = prev->sched_class == &ext_sched_class; + bool keep_prev = rq->scx.flags & SCX_RQ_BAL_KEEP; + bool kick_idle = false; /* - * If balance_scx() is telling us to keep running @prev, replenish slice - * if necessary and keep running @prev. Otherwise, pop the first one - * from the local DSQ. - * * WORKAROUND: * * %SCX_RQ_BAL_KEEP should be set iff $prev is on SCX as it must just @@ -2962,22 +2961,41 @@ static struct task_struct *pick_task_scx(struct rq *rq) * which then ends up calling pick_task_scx() without preceding * balance_scx(). * - * For now, ignore cases where $prev is not on SCX. This isn't great and - * can theoretically lead to stalls. However, for switch_all cases, this - * happens only while a BPF scheduler is being loaded or unloaded, and, - * for partial cases, fair will likely keep triggering this CPU. + * Keep running @prev if possible and avoid stalling from entering idle + * without balancing. * - * Once fair is fixed, restore WARN_ON_ONCE(). + * Once fair is fixed, remove the workaround and trigger WARN_ON_ONCE() + * if pick_task_scx() is called without preceding balance_scx(). */ - if ((rq->scx.flags & SCX_RQ_BAL_KEEP) && - prev->sched_class == &ext_sched_class) { + if (unlikely(rq->scx.flags & SCX_RQ_BAL_PENDING)) { + if (prev_on_scx) { + keep_prev = true; + } else { + keep_prev = false; + kick_idle = true; + } + } else if (unlikely(keep_prev && !prev_on_scx)) { + /* only allowed during transitions */ + WARN_ON_ONCE(scx_ops_enable_state() == SCX_OPS_ENABLED); + keep_prev = false; + } + + /* + * If balance_scx() is telling us to keep running @prev, replenish slice + * if necessary and keep running @prev. Otherwise, pop the first one + * from the local DSQ. + */ + if (keep_prev) { p = prev; if (!p->scx.slice) p->scx.slice = SCX_SLICE_DFL; } else { p = first_local_task(rq); - if (!p) + if (!p) { + if (kick_idle) + scx_bpf_kick_cpu(cpu_of(rq), SCX_KICK_IDLE); return NULL; + } if (unlikely(!p->scx.slice)) { if (!scx_rq_bypassing(rq) && !scx_warned_zero_slice) { diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 6085ef50febf..4d79804631e4 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -751,8 +751,9 @@ enum scx_rq_flags { */ SCX_RQ_ONLINE = 1 << 0, SCX_RQ_CAN_STOP_TICK = 1 << 1, - SCX_RQ_BAL_KEEP = 1 << 2, /* balance decided to keep current */ - SCX_RQ_BYPASSING = 1 << 3, + SCX_RQ_BAL_PENDING = 1 << 2, /* balance hasn't run yet */ + SCX_RQ_BAL_KEEP = 1 << 3, /* balance decided to keep current */ + SCX_RQ_BYPASSING = 1 << 4, SCX_RQ_IN_WAKEUP = 1 << 16, SCX_RQ_IN_BALANCE = 1 << 17,