From 260e1e4fd826a8e47c8976efba6a54d62b4a57de Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Thu, 29 Nov 2018 13:28:49 -0800 Subject: [PATCH] rcu: Discard separate per-CPU callback counts Back when there were multiple flavors of RCU, it was necessary to separately count lazy and non-lazy callbacks for each CPU. These counts were used in CONFIG_RCU_FAST_NO_HZ kernels to determine how long a newly idle CPU should be allowed to sleep before handling its RCU callbacks. But now that there is only one flavor, the callback counts for a given CPU's sole rcu_data structure are the counts for that CPU. This commit therefore removes the rcu_data structure's ->nonlazy_posted and ->nonlazy_posted_snap fields, the rcu_idle_count_callbacks_posted() and rcu_cpu_has_callbacks() functions, repurposes the rcu_data structure's ->all_lazy field to record the laziness state at the beginning of the latest idle sojourn, and modifies CONFIG_RCU_FAST_NO_HZ RCU CPU stall warnings accordingly. Signed-off-by: Paul E. McKenney --- Documentation/RCU/stallwarn.txt | 15 +++++----- kernel/rcu/tree.c | 25 ----------------- kernel/rcu/tree.h | 6 +--- kernel/rcu/tree_plugin.h | 50 ++++++++------------------------- 4 files changed, 21 insertions(+), 75 deletions(-) diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt index 073dbc12d1ea..1ab70c37921f 100644 --- a/Documentation/RCU/stallwarn.txt +++ b/Documentation/RCU/stallwarn.txt @@ -219,17 +219,18 @@ an estimate of the total number of RCU callbacks queued across all CPUs In kernels with CONFIG_RCU_FAST_NO_HZ, more information is printed for each CPU: - 0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 nonlazy_posted: 25 .D + 0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 Nonlazy posted: ..D The "last_accelerate:" prints the low-order 16 bits (in hex) of the jiffies counter when this CPU last invoked rcu_try_advance_all_cbs() from rcu_needs_cpu() or last invoked rcu_accelerate_cbs() from -rcu_prepare_for_idle(). The "nonlazy_posted:" prints the number -of non-lazy callbacks posted since the last call to rcu_needs_cpu(). -Finally, an "L" indicates that there are currently no non-lazy callbacks -("." is printed otherwise, as shown above) and "D" indicates that -dyntick-idle processing is enabled ("." is printed otherwise, for example, -if disabled via the "nohz=" kernel boot parameter). +rcu_prepare_for_idle(). The "Nonlazy posted:" indicates lazy-callback +status, so that an "l" indicates that all callbacks were lazy at the start +of the last idle period and an "L" indicates that there are currently +no non-lazy callbacks (in both cases, "." is printed otherwise, as +shown above) and "D" indicates that dyntick-idle processing is enabled +("." is printed otherwise, for example, if disabled via the "nohz=" +kernel boot parameter). If the grace period ends just as the stall warning starts printing, there will be a spurious stall-warning message, which will include diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index e2bd42b2b563..e53a586b397b 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -2878,9 +2878,6 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func, int cpu, bool lazy) rcu_segcblist_init(&rdp->cblist); } rcu_segcblist_enqueue(&rdp->cblist, head, lazy); - if (!lazy) - rcu_idle_count_callbacks_posted(); - if (__is_kfree_rcu_offset((unsigned long)func)) trace_rcu_kfree_callback(rcu_state.name, head, (unsigned long)func, @@ -3110,28 +3107,6 @@ static int rcu_pending(void) return 0; } -/* - * Return true if the specified CPU has any callback. If all_lazy is - * non-NULL, store an indication of whether all callbacks are lazy. - * (If there are no callbacks, all of them are deemed to be lazy.) - */ -static bool rcu_cpu_has_callbacks(bool *all_lazy) -{ - bool al = true; - bool hc = false; - struct rcu_data *rdp; - - rdp = this_cpu_ptr(&rcu_data); - if (!rcu_segcblist_empty(&rdp->cblist)) { - hc = true; - if (rcu_segcblist_n_nonlazy_cbs(&rdp->cblist)) - al = false; - } - if (all_lazy) - *all_lazy = al; - return hc; -} - /* * Helper function for rcu_barrier() tracing. If tracing is disabled, * the compiler is expected to optimize this away. diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h index 50bb41cdc5fb..20feecbb0ab6 100644 --- a/kernel/rcu/tree.h +++ b/kernel/rcu/tree.h @@ -193,10 +193,7 @@ struct rcu_data { bool rcu_need_heavy_qs; /* GP old, so heavy quiescent state! */ bool rcu_urgent_qs; /* GP old need light quiescent state. */ #ifdef CONFIG_RCU_FAST_NO_HZ - bool all_lazy; /* Are all CPU's CBs lazy? */ - unsigned long nonlazy_posted; /* # times non-lazy CB posted to CPU. */ - unsigned long nonlazy_posted_snap; - /* Nonlazy_posted snapshot. */ + bool all_lazy; /* All CPU's CBs lazy at idle start? */ unsigned long last_accelerate; /* Last jiffy CBs were accelerated. */ unsigned long last_advance_all; /* Last jiffy CBs were all advanced. */ int tick_nohz_enabled_snap; /* Previously seen value from sysfs. */ @@ -430,7 +427,6 @@ static void __init rcu_spawn_boost_kthreads(void); static void rcu_prepare_kthreads(int cpu); static void rcu_cleanup_after_idle(void); static void rcu_prepare_for_idle(void); -static void rcu_idle_count_callbacks_posted(void); static bool rcu_preempt_has_tasks(struct rcu_node *rnp); static bool rcu_preempt_need_deferred_qs(struct task_struct *t); static void rcu_preempt_deferred_qs(struct task_struct *t); diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 43f3f2ee9d63..abd238c70900 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -1474,7 +1474,7 @@ static void rcu_prepare_kthreads(int cpu) int rcu_needs_cpu(u64 basemono, u64 *nextevt) { *nextevt = KTIME_MAX; - return rcu_cpu_has_callbacks(NULL); + return !rcu_segcblist_empty(&this_cpu_ptr(&rcu_data)->cblist); } /* @@ -1493,14 +1493,6 @@ static void rcu_prepare_for_idle(void) { } -/* - * Don't bother keeping a running count of the number of RCU callbacks - * posted because CONFIG_RCU_FAST_NO_HZ=n. - */ -static void rcu_idle_count_callbacks_posted(void) -{ -} - #else /* #if !defined(CONFIG_RCU_FAST_NO_HZ) */ /* @@ -1583,11 +1575,8 @@ int rcu_needs_cpu(u64 basemono, u64 *nextevt) lockdep_assert_irqs_disabled(); - /* Snapshot to detect later posting of non-lazy callback. */ - rdp->nonlazy_posted_snap = rdp->nonlazy_posted; - /* If no callbacks, RCU doesn't need the CPU. */ - if (!rcu_cpu_has_callbacks(&rdp->all_lazy)) { + if (rcu_segcblist_empty(&rdp->cblist)) { *nextevt = KTIME_MAX; return 0; } @@ -1601,11 +1590,12 @@ int rcu_needs_cpu(u64 basemono, u64 *nextevt) rdp->last_accelerate = jiffies; /* Request timer delay depending on laziness, and round. */ - if (!rdp->all_lazy) { + rdp->all_lazy = !rcu_segcblist_n_nonlazy_cbs(&rdp->cblist); + if (rdp->all_lazy) { + dj = round_jiffies(rcu_idle_lazy_gp_delay + jiffies) - jiffies; + } else { dj = round_up(rcu_idle_gp_delay + jiffies, rcu_idle_gp_delay) - jiffies; - } else { - dj = round_jiffies(rcu_idle_lazy_gp_delay + jiffies) - jiffies; } *nextevt = basemono + dj * TICK_NSEC; return 0; @@ -1635,7 +1625,7 @@ static void rcu_prepare_for_idle(void) /* Handle nohz enablement switches conservatively. */ tne = READ_ONCE(tick_nohz_active); if (tne != rdp->tick_nohz_enabled_snap) { - if (rcu_cpu_has_callbacks(NULL)) + if (!rcu_segcblist_empty(&rdp->cblist)) invoke_rcu_core(); /* force nohz to see update. */ rdp->tick_nohz_enabled_snap = tne; return; @@ -1648,10 +1638,8 @@ static void rcu_prepare_for_idle(void) * callbacks, invoke RCU core for the side-effect of recalculating * idle duration on re-entry to idle. */ - if (rdp->all_lazy && - rdp->nonlazy_posted != rdp->nonlazy_posted_snap) { + if (rdp->all_lazy && rcu_segcblist_n_nonlazy_cbs(&rdp->cblist)) { rdp->all_lazy = false; - rdp->nonlazy_posted_snap = rdp->nonlazy_posted; invoke_rcu_core(); return; } @@ -1687,19 +1675,6 @@ static void rcu_cleanup_after_idle(void) invoke_rcu_core(); } -/* - * Keep a running count of the number of non-lazy callbacks posted - * on this CPU. This running counter (which is never decremented) allows - * rcu_prepare_for_idle() to detect when something out of the idle loop - * posts a callback, even if an equal number of callbacks are invoked. - * Of course, callbacks should only be posted from within a trace event - * designed to be called from idle or from within RCU_NONIDLE(). - */ -static void rcu_idle_count_callbacks_posted(void) -{ - __this_cpu_add(rcu_data.nonlazy_posted, 1); -} - #endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */ #ifdef CONFIG_RCU_FAST_NO_HZ @@ -1707,13 +1682,12 @@ static void rcu_idle_count_callbacks_posted(void) static void print_cpu_stall_fast_no_hz(char *cp, int cpu) { struct rcu_data *rdp = &per_cpu(rcu_data, cpu); - unsigned long nlpd = rdp->nonlazy_posted - rdp->nonlazy_posted_snap; - sprintf(cp, "last_accelerate: %04lx/%04lx, nonlazy_posted: %ld, %c%c", + sprintf(cp, "last_accelerate: %04lx/%04lx, Nonlazy posted: %c%c%c", rdp->last_accelerate & 0xffff, jiffies & 0xffff, - ulong2long(nlpd), - rdp->all_lazy ? 'L' : '.', - rdp->tick_nohz_enabled_snap ? '.' : 'D'); + ".l"[rdp->all_lazy], + ".L"[!rcu_segcblist_n_nonlazy_cbs(&rdp->cblist)], + ".D"[!rdp->tick_nohz_enabled_snap]); } #else /* #ifdef CONFIG_RCU_FAST_NO_HZ */