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Message-ID: <6f1d2e1d-5f5f-1727-ec45-abeb4a104753@quicinc.com>
Date: Thu, 30 Jun 2022 12:36:35 +0530
From: Neeraj Upadhyay <quic_neeraju@...cinc.com>
To: Zhangfei Gao <zhangfei.gao@...aro.org>, <paulmck@...nel.org>,
<frederic@...nel.org>, <josh@...htriplett.org>,
<rostedt@...dmis.org>, <mathieu.desnoyers@...icios.com>,
<jiangshanlai@...il.com>, <joel@...lfernandes.org>
CC: <linux-kernel@...r.kernel.org>, <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 v2] srcu: Reduce blocking agressiveness of expedited grace
periods further
On 6/30/2022 12:04 PM, Zhangfei Gao wrote:
>
>
> On 2022/6/30 下午12:12, 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 and Shameer
>> 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, such that, non-sleeping polls are done for one jiffy. In addition
>> 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>
>> Tested-by: Marc Zyngier <maz@...nel.org>
>
> Tested-by: Zhangfei Gao <zhangfei.gao@...aro.org>
Thanks for sharing the test results!! will include your Tested-by tag in
next version.
Thanks
Neeraj
>
> Test on arm64, defconfig(CONFIG_HZ_250=y)
> qemu boot Image with -bios QEMU_EFI.fd
>
>
> With this patch
> real 0m9.739s
> user 0m3.270s
> sys 0m0.969s
>
>
> Without this patch
> real 2m40.361s
> user 0m3.034s
> sys 0m1.162s
>
> 5.18-rc6
> real 0m8.402s
> user 0m3.015s
> sys 0m1.102s
>
> Thanks
>
>> ---
>>
>> Change in v2:
>>
>> - Change srcu_max_nodelay default value to consider phase delay
>> iterations
>> - Apply Pauls' feedback
>> - Add Marc's Tested-by
>>
>> .../admin-guide/kernel-parameters.txt | 18 ++++
>> kernel/rcu/srcutree.c | 82 ++++++++++++++-----
>> 2 files changed, 81 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..1c304fec89c0 100644
>> --- a/kernel/rcu/srcutree.c
>> +++ b/kernel/rcu/srcutree.c
>> @@ -511,10 +511,52 @@ 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))
>> +#define SRCU_UL_CLAMP(val, low, high)
>> SRCU_UL_CLAMP_HI(SRCU_UL_CLAMP_LO((val), (low)), (high))
>> +// 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(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);
>> +
>> +// Maximum consecutive no-delay instances.
>> +#define SRCU_DEFAULT_MAX_NODELAY (SRCU_DEFAULT_MAX_NODELAY_PHASE >
>> 100 ? \
>> + SRCU_DEFAULT_MAX_NODELAY_PHASE : 100)
>> +
>> +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 +577,7 @@ 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 +654,6 @@ 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 +739,7 @@ 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 +759,9 @@ 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 +962,16 @@ 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 +1627,7 @@ 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 +1719,11 @@ 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);
>
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