On Mon, Jun 13, 2022 at 08:26:34PM +0800, chenxiang (M) wrote:
Hi all,
I encounter a issue with kernel 5.19-rc1 on a ARM64 board: it takes about
150s between beginning to run qemu command and beginng to boot Linux kernel
("EFI stub: Booting Linux Kernel...").
But in kernel 5.18-rc4, it only takes about 5s. I git bisect the kernel code
and it finds c2445d387850 ("srcu: Add contention check to call_srcu()
srcu_data ->lock acquisition").
The qemu (qemu version is 6.2.92) command i run is :
./qemu-system-aarch64 -m 4G,slots=4,maxmem=8g \
--trace "kvm*" \
-cpu host \
-machine virt,accel=kvm,gic-version=3 \
-machine smp.cpus=2,smp.sockets=2 \
-no-reboot \
-nographic \
-monitor unix:/home/cx/qmp-test,server,nowait \
-bios /home/cx/boot/QEMU_EFI.fd \
-kernel /home/cx/boot/Image \
-device
pcie-root-port,port=0x8,chassis=1,id=net1,bus=pcie.0,multifunction=on,addr=0x1
\
-device vfio-pci,host=7d:01.3,id=net0 \
-device virtio-blk-pci,drive=drive0,id=virtblk0,num-queues=4 \
-drive file=/home/cx/boot/boot_ubuntu.img,if=none,id=drive0 \
-append "rdinit=init console=ttyAMA0 root=/dev/vda rootfstype=ext4 rw " \
-net none \
-D /home/cx/qemu_log.txt
I am not familiar with rcu code, and don't know how it causes the issue. Do
you have any idea about this issue?
Please see the discussion here:
https://lore.kernel.org/all/20615615-0013-5adc-584f-2b1d5c03ebfc@linaro.org/
Though that report requires ACPI to be forced on to get the
delay, which results in more than 9,000 back-to-back calls to
synchronize_srcu_expedited(). I cannot reproduce this on my setup, even
with an artificial tight loop invoking synchronize_srcu_expedited(),
but then again I don't have ARM hardware.
My current guess is that the following patch, but with larger values for
SRCU_MAX_NODELAY_PHASE. Here "larger" might well be up in the hundreds,
or perhaps even larger.
If you get a chance to experiment with this, could you please reply
to the discussion at the above URL? (Or let me know, and I can CC
you on the next message in that thread.)
Thanx, Paul
------------------------------------------------------------------------
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;
}
.