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Message-ID: <a3603e76-82b5-529d-d1a9-54f7b5214af8@quicinc.com>
Date:   Tue, 28 Jun 2022 08:52:05 +0530
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Subject: Re: [PATCH] srcu: Reduce blocking agressiveness of expedited grace
 periods further
Content-Language: en-US
To:     Zhangfei Gao <zhangfei.gao@linaro.org>, <paulmck@kernel.org>,
        <frederic@kernel.org>, <josh@joshtriplett.org>,
        <rostedt@goodmis.org>, <mathieu.desnoyers@efficios.com>,
        <jiangshanlai@gmail.com>, <joel@joelfernandes.org>
CC:     <linux-kernel@vger.kernel.org>, <boqun.feng@gmail.com>,
        <urezki@gmail.com>, <shameerali.kolothum.thodi@huawei.com>,
        <pbonzini@redhat.com>, <mtosatti@redhat.com>,
        <eric.auger@redhat.com>, <chenxiang66@hisilicon.com>,
        <maz@kernel.org>
References: <20220627123706.20187-1-quic_neeraju@quicinc.com>
 <60336a5f-e7ab-48e6-2076-8db4ceb0561a@linaro.org>
From:   Neeraj Upadhyay <quic_neeraju@quicinc.com>
In-Reply-To: <60336a5f-e7ab-48e6-2076-8db4ceb0561a@linaro.org>
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On 6/28/2022 7:44 AM, Zhangfei Gao wrote:
> 
> 
> On 2022/6/27 下午8:37, 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,
>> 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
>> 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@quicinc.com>
> 
> Test on arm64
> git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git dev
> 5.19-rc3
> 
> arch/arm64/configs/defconfig make defconfig CONFIG_PREEMPTION=y 
> CONFIG_HZ_250=y
> 

If it is possible to try out, do you get similar results with HZ_1000?

> real    0m11.498s
> user    0m2.911s
> sys    0m1.171s
> 
> 
> As comparison (Since can not directly revert on linux-rcu.git dev, so 
> use rc1 instead)
> 
> 5.19-rc1 + Revert "srcu: Prevent expedited GPs and blocking readers from 
> consuming CPU"
> 
> real    0m8.173s
> user    0m3.024s
> sys    0m0.959s
> 
> 5.19-rc1
> real    2m41.433s
>   user    0m3.097s
> sys     0m1.177s
> 

Thanks! Can I add your Tested-by in subsequent versions of the patch?

The numbers are aligned to the initial experiments, without using long 
retry delays of 100 us. Using long delays might have impact on other
workloads, which could be sensitive to the delay between retries. So,
I didn't include that in the patch.


Thanks
Neeraj

> 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))
>> +// 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);
>