> -----Original Message----- > From: Paul E. McKenney [mailto:paulmck@xxxxxxxxxx] > Sent: 20 June 2022 23:21 > To: rcu@xxxxxxxxxxxxxxx > Cc: linux-kernel@xxxxxxxxxxxxxxx; kernel-team@xxxxxx; > rostedt@xxxxxxxxxxx; Paul E. McKenney <paulmck@xxxxxxxxxx>; Zhangfei > Gao <zhangfei.gao@xxxxxxxxxx>; Shameerali Kolothum Thodi > <shameerali.kolothum.thodi@xxxxxxxxxx>; Paolo Bonzini > <pbonzini@xxxxxxxxxx> > Subject: [PATCH rcu 12/12] srcu: Block less aggressively for expedited grace > periods > > 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. As pointed out here[0]/[1], this delay has nothing to do with ARM IORT RMR feature. The delay is due to the "-bios QEMU_EFI.fd" line which emulates flash devices and requires excessive memslot ops during boot. Thanks, Shameer [0] https://lore.kernel.org/all/110bbec5cee74efba0aad64360069a12@xxxxxxxxxx/ [1] https://lore.kernel.org/all/8735g649ew.wl-maz@xxxxxxxxxx/ 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@linaro > .org/ > --- > 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; > } > -- > 2.31.1.189.g2e36527f23
