On Tue, Jun 21, 2022 at 07:43:08AM +0000, Shameerali Kolothum Thodi wrote: > > > > -----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. Good eyes, and I will update. Are we stuck with the excessive memslot ops? If not, great. If we are stuck with them, there probably needs to be a way of adjusting the SRCU delay parameters. Making "-bios QEMU_EFI.fd" go quickly seems to require quite a bit of spinning, more than is likely to be helpful in general. Thanx, Paul > 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 >
