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

Zhangfei Gao
2022-06-21T02:00:28Z

On 2022/6/21 上午6:20, 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/

Test on 5.19-rc1 with this patch with qemu boot with -bios
QEMU_EFI-2022.fd, seems not work, same as rc1.

real    2m42.948s
user    0m2.843s
sys     0m1.170s

qemu: stable-6.1

build/aarch64-softmmu/qemu-system-aarch64 -machine
virt,gic-version=3,iommu=smmuv3 \
-enable-kvm -cpu host -m 1024 \
-kernel /home/linaro/Image -initrd /home/linaro/tmp/ramdisk-new.img
-nographic -append \
"rdinit=init console=ttyAMA0 earlycon=pl011,0x9000000 kpti=off acpi=force" \
-bios QEMU_EFI-2022.fd

Thanks
> ---
> 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;
> }