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

[Paul E. McKenney]

On Tue, Jun 21, 2022 at 03:43:30PM +0530, Neeraj Upadhyay wrote:
> 
> 
> 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@xxxxxxxxxx>
> > Reported-by: Zhangfei Gao <zhangfei.gao@xxxxxxxxxx>
> > Reported-by: chenxiang (M)" <chenxiang66@xxxxxxxxxxxxx>
> > Cc: Shameerali Kolothum Thodi  <shameerali.kolothum.thodi@xxxxxxxxxx>
> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Link: https://lore.kernel.org/all/20615615-0013-5adc-584f-2b1d5c03ebfc@xxxxxxxxxx/
> > ---
> 
> 
> Reviewed-by: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>

And I applied your Reviewed-by to the other commits you gave it for,
and thank you very much!  Much nicer to fix the bugs now than to have
to do separate patches for them later.  ;-)

							Thanx, Paul

> 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;
> >   }