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Date: Tue, 28 Jun 2022 08:45:50 +0530 From: Neeraj Upadhyay <quic_neeraju@...cinc.com> To: <paulmck@...nel.org> CC: <frederic@...nel.org>, <josh@...htriplett.org>, <rostedt@...dmis.org>, <mathieu.desnoyers@...icios.com>, <jiangshanlai@...il.com>, <joel@...lfernandes.org>, <linux-kernel@...r.kernel.org>, <zhangfei.gao@...mail.com>, <boqun.feng@...il.com>, <urezki@...il.com>, <shameerali.kolothum.thodi@...wei.com>, <pbonzini@...hat.com>, <mtosatti@...hat.com>, <eric.auger@...hat.com>, <chenxiang66@...ilicon.com>, <maz@...nel.org> Subject: Re: [PATCH] srcu: Reduce blocking agressiveness of expedited grace periods further On 6/28/2022 4:15 AM, Paul E. McKenney wrote: > On Mon, Jun 27, 2022 at 06:07:06PM +0530, Neeraj Upadhyay wrote: >> Commit 640a7d37c3f4 ("srcu: Block less aggressively for expedited >> grace periods") highlights a problem where aggressively blocking >> SRCU expedited grace periods, as was introduced in commit >> 282d8998e997 ("srcu: Prevent expedited GPs and blocking readers >> from consuming CPU"), introduces ~2 minutes delay to the overall >> ~3.5 minutes boot time, when starting VMs with "-bios QEMU_EFI.fd" >> cmdline on qemu, which results in very high rate of memslots >> add/remove, which causes > ~6000 synchronize_srcu() calls for >> kvm->srcu SRCU instance. >> >> Below table captures the experiments done by Zhangfei Gao, Shameer, > > Zhangfei Gao and Shameerali Kolothum without commas? > Will fix in next version >> to measure the boottime impact with various values of non-sleeping >> per phase counts, with HZ_250 and preemption enabled: >> >> +──────────────────────────+────────────────+ >> | SRCU_MAX_NODELAY_PHASE | Boot time (s) | >> +──────────────────────────+────────────────+ >> | 100 | 30.053 | >> | 150 | 25.151 | >> | 200 | 20.704 | >> | 250 | 15.748 | >> | 500 | 11.401 | >> | 1000 | 11.443 | >> | 10000 | 11.258 | >> | 1000000 | 11.154 | >> +──────────────────────────+────────────────+ >> >> Analysis on the experiment results showed improved boot time >> with non blocking delays close to one jiffy duration. This >> was also seen when number of per-phase iterations were scaled >> to one jiffy. >> >> So, this change scales per-grace-period phase number of non-sleeping >> polls, soiuch that, non-sleeping polls are done for one jiffy. In addition > > such that? > oops, will fix. >> to this, srcu_get_delay() call in srcu_gp_end(), which is used to calculate >> the delay used for scheduling callbacks, is replaced with the check for >> expedited grace period. This is done, to schedule cbs for completed expedited >> grace periods immediately, which results in improved boot time seen in >> experiments. >> >> In addition to the changes to default per phase delays, this change >> adds 3 new kernel parameters - srcutree.srcu_max_nodelay, >> srcutree.srcu_max_nodelay_phase, srcutree.srcu_retry_check_delay. >> This allows users to configure the srcu grace period scanning delays, >> depending on their system configuration requirements. >> >> Signed-off-by: Neeraj Upadhyay <quic_neeraju@...cinc.com> > > I have queued this on an experimental branch quic_neeraju.2022.06.27a > for testing purposes. One question below. Thanks! > >> --- >> .../admin-guide/kernel-parameters.txt | 18 +++++ >> kernel/rcu/srcutree.c | 79 ++++++++++++++----- >> 2 files changed, 78 insertions(+), 19 deletions(-) >> >> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt >> index af647714c113..7e34086c64f5 100644 >> --- a/Documentation/admin-guide/kernel-parameters.txt >> +++ b/Documentation/admin-guide/kernel-parameters.txt >> @@ -5805,6 +5805,24 @@ >> expediting. Set to zero to disable automatic >> expediting. >> >> + srcutree.srcu_max_nodelay [KNL] >> + Specifies the number of no-delay instances >> + per jiffy for which the SRCU grace period >> + worker thread will be rescheduled with zero >> + delay. Beyond this limit, worker thread will >> + be rescheduled with a sleep delay of one jiffy. >> + >> + srcutree.srcu_max_nodelay_phase [KNL] >> + Specifies the per-grace-period phase, number of >> + non-sleeping polls of readers. Beyond this limit, >> + grace period worker thread will be rescheduled >> + with a sleep delay of one jiffy, between each >> + rescan of the readers, for a grace period phase. >> + >> + srcutree.srcu_retry_check_delay [KNL] >> + Specifies number of microseconds of non-sleeping >> + delay between each non-sleeping poll of readers. >> + >> srcutree.small_contention_lim [KNL] >> Specifies the number of update-side contention >> events per jiffy will be tolerated before >> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c >> index 0db7873f4e95..006828b9c41a 100644 >> --- a/kernel/rcu/srcutree.c >> +++ b/kernel/rcu/srcutree.c >> @@ -511,10 +511,49 @@ static bool srcu_readers_active(struct srcu_struct *ssp) >> return sum; >> } >> >> -#define SRCU_INTERVAL 1 // Base delay if no expedited GPs pending. >> -#define SRCU_MAX_INTERVAL 10 // Maximum incremental delay from slow readers. >> -#define SRCU_MAX_NODELAY_PHASE 3 // Maximum per-GP-phase consecutive no-delay instances. >> -#define SRCU_MAX_NODELAY 100 // Maximum consecutive no-delay instances. >> +/* >> + * We use an adaptive strategy for synchronize_srcu() and especially for >> + * synchronize_srcu_expedited(). We spin for a fixed time period >> + * (defined below, boot time configurable) to allow SRCU readers to exit >> + * their read-side critical sections. If there are still some readers >> + * after one jiffy, we repeatedly block for one jiffy time periods. >> + * The blocking time is increased as the grace-period age increases, >> + * with max blocking time capped at 10 jiffies. >> + */ >> +#define SRCU_DEFAULT_RETRY_CHECK_DELAY 5 >> + >> +static ulong srcu_retry_check_delay = SRCU_DEFAULT_RETRY_CHECK_DELAY; >> +module_param(srcu_retry_check_delay, ulong, 0444); >> + >> +#define SRCU_INTERVAL 1 // Base delay if no expedited GPs pending. >> +#define SRCU_MAX_INTERVAL 10 // Maximum incremental delay from slow readers. >> + >> +#define SRCU_DEFAULT_MAX_NODELAY_PHASE_LO 3UL // Lowmark on default per-GP-phase >> + // no-delay instances. >> +#define SRCU_DEFAULT_MAX_NODELAY_PHASE_HI 1000UL // Highmark on default per-GP-phase >> + // no-delay instances. >> + >> +#define SRCU_UL_CLAMP_LO(val, low) ((val) > (low) ? (val) : (low)) >> +#define SRCU_UL_CLAMP_HI(val, high) ((val) < (high) ? (val) : (high)) > > Can these just be clamp()? Or does its type checking get in the way? > When I use clamp() or clamp_t(), I get warning like below: CC kernel/rcu/srcutree.o In file included from ./include/linux/kernel.h:26:0, from ./arch/x86/include/asm/percpu.h:27, from ./arch/x86/include/asm/current.h:6, from ./include/linux/mutex.h:14, from kernel/rcu/srcutree.c:19: ./include/linux/minmax.h:30:50: error: braced-group within expression allowed only inside a function #define __cmp_once(x, y, unique_x, unique_y, op) ({ \ ^ ./include/linux/minmax.h:20:21: note: in definition of macro ‘__typecheck’ (!!(sizeof((typeof(x) *)1 == (typeof(y) *)1))) Which seems to be coming from braced usage in __cmp_once() #define __cmp_once(x, y, unique_x, unique_y, op) ({ \ typeof(x) unique_x = (x); \ typeof(y) unique_y = (y); \ __cmp(unique_x, unique_y, op); }) ... as I am using clamp() to initialize module param , which is outside of a function. static ulong srcu_max_nodelay_phase = SRCU_DEFAULT_MAX_NODELAY_PHASE; Thanks Neeraj > Thanx, Paul > >> +// per-GP-phase no-delay instances adjusted to allow non-sleeping poll upto >> +// one jiffies time duration. Mult by 2 is done to factor in the srcu_get_delay() >> +// called from process_srcu(). >> +#define SRCU_DEFAULT_MAX_NODELAY_PHASE_ADJUSTED \ >> + (2UL * USEC_PER_SEC / HZ / SRCU_DEFAULT_RETRY_CHECK_DELAY) >> + >> +// Maximum per-GP-phase consecutive no-delay instances. >> +#define SRCU_DEFAULT_MAX_NODELAY_PHASE ( \ >> + SRCU_UL_CLAMP_HI(SRCU_UL_CLAMP_LO(SRCU_DEFAULT_MAX_NODELAY_PHASE_ADJUSTED, \ >> + SRCU_DEFAULT_MAX_NODELAY_PHASE_LO), \ >> + SRCU_DEFAULT_MAX_NODELAY_PHASE_HI)) >> + >> +static ulong srcu_max_nodelay_phase = SRCU_DEFAULT_MAX_NODELAY_PHASE; >> +module_param(srcu_max_nodelay_phase, ulong, 0444); >> + >> +#define SRCU_DEFAULT_MAX_NODELAY 100 // Maximum consecutive no-delay instances. >> + >> +static ulong srcu_max_nodelay = SRCU_DEFAULT_MAX_NODELAY; >> +module_param(srcu_max_nodelay, ulong, 0444); >> >> /* >> * Return grace-period delay, zero if there are expedited grace >> @@ -535,7 +574,7 @@ static unsigned long srcu_get_delay(struct srcu_struct *ssp) >> jbase += j - gpstart; >> if (!jbase) { >> WRITE_ONCE(ssp->srcu_n_exp_nodelay, READ_ONCE(ssp->srcu_n_exp_nodelay) + 1); >> - if (READ_ONCE(ssp->srcu_n_exp_nodelay) > SRCU_MAX_NODELAY_PHASE) >> + if (READ_ONCE(ssp->srcu_n_exp_nodelay) > srcu_max_nodelay_phase) >> jbase = 1; >> } >> } >> @@ -612,15 +651,6 @@ void __srcu_read_unlock(struct srcu_struct *ssp, int idx) >> } >> EXPORT_SYMBOL_GPL(__srcu_read_unlock); >> >> -/* >> - * We use an adaptive strategy for synchronize_srcu() and especially for >> - * synchronize_srcu_expedited(). We spin for a fixed time period >> - * (defined below) to allow SRCU readers to exit their read-side critical >> - * sections. If there are still some readers after a few microseconds, >> - * we repeatedly block for 1-millisecond time periods. >> - */ >> -#define SRCU_RETRY_CHECK_DELAY 5 >> - >> /* >> * Start an SRCU grace period. >> */ >> @@ -706,7 +736,7 @@ static void srcu_schedule_cbs_snp(struct srcu_struct *ssp, struct srcu_node *snp >> */ >> static void srcu_gp_end(struct srcu_struct *ssp) >> { >> - unsigned long cbdelay; >> + unsigned long cbdelay = 1; >> bool cbs; >> bool last_lvl; >> int cpu; >> @@ -726,7 +756,9 @@ static void srcu_gp_end(struct srcu_struct *ssp) >> spin_lock_irq_rcu_node(ssp); >> idx = rcu_seq_state(ssp->srcu_gp_seq); >> WARN_ON_ONCE(idx != SRCU_STATE_SCAN2); >> - cbdelay = !!srcu_get_delay(ssp); >> + if (ULONG_CMP_LT(READ_ONCE(ssp->srcu_gp_seq), READ_ONCE(ssp->srcu_gp_seq_needed_exp))) >> + cbdelay = 0; >> + >> WRITE_ONCE(ssp->srcu_last_gp_end, ktime_get_mono_fast_ns()); >> rcu_seq_end(&ssp->srcu_gp_seq); >> gpseq = rcu_seq_current(&ssp->srcu_gp_seq); >> @@ -927,12 +959,16 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp, >> */ >> static bool try_check_zero(struct srcu_struct *ssp, int idx, int trycount) >> { >> + unsigned long curdelay; >> + >> + curdelay = !srcu_get_delay(ssp); >> + >> for (;;) { >> if (srcu_readers_active_idx_check(ssp, idx)) >> return true; >> - if (--trycount + !srcu_get_delay(ssp) <= 0) >> + if ((--trycount + curdelay) <= 0) >> return false; >> - udelay(SRCU_RETRY_CHECK_DELAY); >> + udelay(srcu_retry_check_delay); >> } >> } >> >> @@ -1588,7 +1624,7 @@ static void process_srcu(struct work_struct *work) >> j = jiffies; >> if (READ_ONCE(ssp->reschedule_jiffies) == j) { >> WRITE_ONCE(ssp->reschedule_count, READ_ONCE(ssp->reschedule_count) + 1); >> - if (READ_ONCE(ssp->reschedule_count) > SRCU_MAX_NODELAY) >> + if (READ_ONCE(ssp->reschedule_count) > srcu_max_nodelay) >> curdelay = 1; >> } else { >> WRITE_ONCE(ssp->reschedule_count, 1); >> @@ -1680,6 +1716,11 @@ static int __init srcu_bootup_announce(void) >> pr_info("Hierarchical SRCU implementation.\n"); >> if (exp_holdoff != DEFAULT_SRCU_EXP_HOLDOFF) >> pr_info("\tNon-default auto-expedite holdoff of %lu ns.\n", exp_holdoff); >> + if (srcu_retry_check_delay != SRCU_DEFAULT_RETRY_CHECK_DELAY) >> + pr_info("\tNon-default retry check delay of %lu us.\n", srcu_retry_check_delay); >> + if (srcu_max_nodelay != SRCU_DEFAULT_MAX_NODELAY) >> + pr_info("\tNon-default max no-delay of %lu.\n", srcu_max_nodelay); >> + pr_info("\tMax phase no-delay instances is %lu.\n", srcu_max_nodelay_phase); >> return 0; >> } >> early_initcall(srcu_bootup_announce); >> -- >> 2.17.1 >>
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