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Date: Thu, 17 Aug 2017 10:22:44 +0200
From: Ingo Molnar <mingo@...nel.org>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: linux-kernel@...r.kernel.org, jiangshanlai@...il.com,
dipankar@...ibm.com, akpm@...ux-foundation.org,
mathieu.desnoyers@...icios.com, josh@...htriplett.org,
tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
oleg@...hat.com, Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v5 tip/core/rcu 01/15] sched,rcu: Make cond_resched()
provide RCU quiescent state
* Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> There is some confusion as to which of cond_resched() or
> cond_resched_rcu_qs() should be added to long in-kernel loops.
> This commit therefore eliminates the decision by adding RCU quiescent
> states to cond_resched(). This commit also simplifies the code that
> used to interact with cond_resched_rcu_qs(), and that now interacts with
> cond_resched(), to reduce its overhead. This reduction is necessary to
> allow the heavier-weight cond_resched_rcu_qs() mechanism to be invoked
> everywhere that cond_resched() is invoked.
>
> Part of that reduction in overhead converts the jiffies_till_sched_qs
> kernel parameter to read-only at runtime, thus eliminating the need for
> bounds checking.
>
> Reported-by: Michal Hocko <mhocko@...nel.org>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> [ paulmck: Tuning for performance issues reported by 0day Test Robot. ]
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 8337e2db0bb2..d2f291a3a44a 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1522,10 +1522,11 @@ static inline int test_tsk_need_resched(struct task_struct *tsk)
> * cond_resched_lock() will drop the spinlock before scheduling,
> * cond_resched_softirq() will enable bhs before scheduling.
> */
> +void rcu_all_qs(void);
> #ifndef CONFIG_PREEMPT
> extern int _cond_resched(void);
> #else
> -static inline int _cond_resched(void) { return 0; }
> +static inline int _cond_resched(void) { rcu_all_qs(); return 0; }
> #endif
>
> #define cond_resched() ({ \
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 51d4c3acf32d..e40cb5190783 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -537,8 +537,8 @@ module_param(rcu_kick_kthreads, bool, 0644);
> * How long the grace period must be before we start recruiting
> * quiescent-state help from rcu_note_context_switch().
> */
> -static ulong jiffies_till_sched_qs = HZ / 20;
> -module_param(jiffies_till_sched_qs, ulong, 0644);
> +static ulong jiffies_till_sched_qs = HZ / 10;
> +module_param(jiffies_till_sched_qs, ulong, 0444);
>
> static bool rcu_start_gp_advanced(struct rcu_state *rsp, struct rcu_node *rnp,
> struct rcu_data *rdp);
> @@ -1230,7 +1230,6 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
> unsigned long jtsq;
> bool *rnhqp;
> bool *ruqp;
> - unsigned long rjtsc;
> struct rcu_node *rnp;
>
> /*
> @@ -1247,23 +1246,13 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
> return 1;
> }
>
> - /* Compute and saturate jiffies_till_sched_qs. */
> - jtsq = jiffies_till_sched_qs;
> - rjtsc = rcu_jiffies_till_stall_check();
> - if (jtsq > rjtsc / 2) {
> - WRITE_ONCE(jiffies_till_sched_qs, rjtsc);
> - jtsq = rjtsc / 2;
> - } else if (jtsq < 1) {
> - WRITE_ONCE(jiffies_till_sched_qs, 1);
> - jtsq = 1;
> - }
> -
> /*
> * Has this CPU encountered a cond_resched_rcu_qs() since the
> * beginning of the grace period? For this to be the case,
> * the CPU has to have noticed the current grace period. This
> * might not be the case for nohz_full CPUs looping in the kernel.
> */
> + jtsq = jiffies_till_sched_qs;
> rnp = rdp->mynode;
> ruqp = per_cpu_ptr(&rcu_dynticks.rcu_urgent_qs, rdp->cpu);
> if (time_after(jiffies, rdp->rsp->gp_start + jtsq) &&
> @@ -1271,7 +1260,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
> READ_ONCE(rdp->gpnum) == rnp->gpnum && !rdp->gpwrap) {
> trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("rqc"));
> return 1;
> - } else {
> + } else if (time_after(jiffies, rdp->rsp->gp_start + jtsq)) {
> /* Load rcu_qs_ctr before store to rcu_urgent_qs. */
> smp_store_release(ruqp, true);
> }
> @@ -1299,10 +1288,6 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
> * updates are only once every few jiffies, the probability of
> * lossage (and thus of slight grace-period extension) is
> * quite low.
> - *
> - * Note that if the jiffies_till_sched_qs boot/sysfs parameter
> - * is set too high, we override with half of the RCU CPU stall
> - * warning delay.
> */
> rnhqp = &per_cpu(rcu_dynticks.rcu_need_heavy_qs, rdp->cpu);
> if (!READ_ONCE(*rnhqp) &&
> @@ -1311,7 +1296,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
> WRITE_ONCE(*rnhqp, true);
> /* Store rcu_need_heavy_qs before rcu_urgent_qs. */
> smp_store_release(ruqp, true);
> - rdp->rsp->jiffies_resched += 5; /* Re-enable beating. */
> + rdp->rsp->jiffies_resched += jtsq; /* Re-enable beating. */
> }
>
> /*
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 17c667b427b4..9433633012ba 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4808,6 +4808,7 @@ int __sched _cond_resched(void)
> preempt_schedule_common();
> return 1;
> }
> + rcu_all_qs();
> return 0;
> }
> EXPORT_SYMBOL(_cond_resched);
So I'm a bit uneasy about this change:
- There's hundreds of uses of cond_resched(), some of them in commonly inlined
functions.
- cond_resched() typically gets called in functions that _might_ take a long time
to execute, but that's not a given.
- it's definitely getting called opportunistically as well, under
PREEMPT_VOLUNTARY, from common lightweight helpers that we know are in
schedulable contexts. We risk adding significant overhead here.
So what we risk here is turning a known to be super simple function into something
much slower - and exporting slowdowns to literally thousands of explicit and
implicit usage sites.
Thanks,
Ingo
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