Re: [PATCH rcu 12/12] srcu: Block less aggressively for expedited grace periods

[Neeraj Upadhyay <quic_neeraju@quicinc.com>]

On 6/21/2022 3:50 AM, Paul E. McKenney wrote:
> Commit 282d8998e997 ("srcu: Prevent expedited GPs and blocking readers
> from consuming CPU") fixed a problem where a long-running expedited SRCU
> grace period could block kernel live patching.  It did so by giving up
> on expediting once a given SRCU expedited grace period grew too old.
> 
> Unfortunately, this added excessive delays to boots of embedded systems
> running on qemu that use the ARM IORT RMR feature.  This commit therefore
> makes the transition away from expediting less aggressive, increasing
> the per-grace-period phase number of non-sleeping polls of readers from
> one to three and increasing the required grace-period age from one jiffy
> (actually from zero to one jiffies) to two jiffies (actually from one
> to two jiffies).
> 
> Fixes: 282d8998e997 ("srcu: Prevent expedited GPs and blocking readers from consuming CPU")
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Reported-by: Zhangfei Gao <zhangfei.gao@linaro.org>
> Reported-by: chenxiang (M)" <chenxiang66@hisilicon.com>
> Cc: Shameerali Kolothum Thodi  <shameerali.kolothum.thodi@huawei.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Link: https://lore.kernel.org/all/20615615-0013-5adc-584f-2b1d5c03ebfc@linaro.org/
> ---


Reviewed-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>


Thanks
Neeraj

>   kernel/rcu/srcutree.c | 20 +++++++++++++-------
>   1 file changed, 13 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 50ba70f019dea..0db7873f4e95b 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -513,7 +513,7 @@ static bool srcu_readers_active(struct srcu_struct *ssp)
>   
>   #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	1	// Maximum per-GP-phase consecutive no-delay instances.
> +#define SRCU_MAX_NODELAY_PHASE	3	// Maximum per-GP-phase consecutive no-delay instances.
>   #define SRCU_MAX_NODELAY	100	// Maximum consecutive no-delay instances.
>   
>   /*
> @@ -522,16 +522,22 @@ static bool srcu_readers_active(struct srcu_struct *ssp)
>    */
>   static unsigned long srcu_get_delay(struct srcu_struct *ssp)
>   {
> +	unsigned long gpstart;
> +	unsigned long j;
>   	unsigned long jbase = SRCU_INTERVAL;
>   
>   	if (ULONG_CMP_LT(READ_ONCE(ssp->srcu_gp_seq), READ_ONCE(ssp->srcu_gp_seq_needed_exp)))
>   		jbase = 0;
> -	if (rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)))
> -		jbase += jiffies - READ_ONCE(ssp->srcu_gp_start);
> -	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)
> -			jbase = 1;
> +	if (rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq))) {
> +		j = jiffies - 1;
> +		gpstart = READ_ONCE(ssp->srcu_gp_start);
> +		if (time_after(j, gpstart))
> +			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)
> +				jbase = 1;
> +		}
>   	}
>   	return jbase > SRCU_MAX_INTERVAL ? SRCU_MAX_INTERVAL : jbase;
>   }