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srcu: Block less aggressively for expedited grace periods

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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 virtual embedded
systems specifying "-bios QEMU_EFI.fd" to qemu.  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>
Reviewed-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Link: https://lore.kernel.org/all/20615615-0013-5adc-584f-2b1d5c03ebfc@linaro.org/
This commit is contained in:
Paul E. McKenney 2022-06-12 15:00:06 -07:00
parent a111daf0c5
commit 8f870e6eb8
1 changed files with 13 additions and 7 deletions
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20
kernel/rcu/srcutree.c
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@ -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;
}