Re: Commit 282d8998e997 (srcu: Prevent expedited GPs and blocking readers from consuming CPU) cause qemu boot slow
From: zhangfei.gao@xxxxxxxxxxx
Date: Fri Jun 24 2022 - 11:32:19 EST
On 2022/6/20 下午3:50, Neeraj Upadhyay wrote:
Hi,
On 6/18/2022 8:37 AM, zhangfei.gao@xxxxxxxxxxx wrote:
On 2022/6/15 下午6:40, Neeraj Upadhyay wrote:
Hi,
On 6/15/2022 2:33 PM, zhangfei.gao@xxxxxxxxxxx wrote:
On 2022/6/14 下午10:17, Paul E. McKenney wrote:
On Tue, Jun 14, 2022 at 10:03:35PM +0800, zhangfei.gao@xxxxxxxxxxx
wrote:
On 2022/6/14 下午8:19, Neeraj Upadhyay wrote:
5.18-rc4 based ~8sec
5.19-rc1 ~2m43sec
5.19-rc1+fix1 ~19sec
5.19-rc1-fix2 ~19sec
If you try below diff on top of either 5.19-rc1+fix1 or
5.19-rc1-fix2 ;
does it show any difference in boot time?
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -706,7 +706,7 @@ static void srcu_schedule_cbs_snp(struct
srcu_struct
*ssp, struct srcu_node *snp
*/
static void srcu_gp_end(struct srcu_struct *ssp)
{
- unsigned long cbdelay;
+ unsigned long cbdelay = 1;
bool cbs;
bool last_lvl;
int cpu;
@@ -726,7 +726,9 @@ static void srcu_gp_end(struct srcu_struct
*ssp)
spin_lock_irq_rcu_node(ssp);
idx = rcu_seq_state(ssp->srcu_gp_seq);
WARN_ON_ONCE(idx != SRCU_STATE_SCAN2);
- cbdelay = !!srcu_get_delay(ssp);
+ if (ULONG_CMP_LT(READ_ONCE(ssp->srcu_gp_seq),
READ_ONCE(ssp->srcu_gp_seq_needed_exp)))
+ cbdelay = 0;
+
WRITE_ONCE(ssp->srcu_last_gp_end,
ktime_get_mono_fast_ns());
Thank you both for the testing and the proposed fix!
Test here:
qemu: https://github.com/qemu/qemu/tree/stable-6.1
kernel:
https://github.com/Linaro/linux-kernel-uadk/tree/uacce-devel-5.19-srcu-test
(in case test patch not clear, push in git tree)
Hardware: aarch64
1. 5.18-rc6
real 0m8.402s
user 0m3.015s
sys 0m1.102s
2. 5.19-rc1
real 2m41.433s
user 0m3.097s
sys 0m1.177s
3. 5.19-rc1 + fix1 from Paul
real 2m43.404s
user 0m2.880s
sys 0m1.214s
4. 5.19-rc1 + fix2: fix1 + Remove "if (!jbase)" block
real 0m15.262s
user 0m3.003s
sys 0m1.033s
When build kernel in the meantime, load time become longer.
5. 5.19-rc1 + fix3: fix1 + SRCU_MAX_NODELAY_PHASE 1000000
real 0m15.215s
user 0m2.942s
sys 0m1.172s
6. 5.19-rc1 + fix4: fix1 + Neeraj's change of srcu_gp_end
real 1m23.936s
user 0m2.969s
sys 0m1.181s
And thank you for the testing!
Could you please try fix3 + Neeraj's change of srcu_gp_end?
That is, fix1 + SRCU_MAX_NODELAY_PHASE 1000000 + Neeraj's change of
srcu_gp_end.
Also, at what value of SRCU_MAX_NODELAY_PHASE do the boot
times start rising? This is probably best done by starting with
SRCU_MAX_NODELAY_PHASE=100000 and dividing by (say) ten on each run
until boot time becomes slow, followed by a binary search between the
last two values. (The idea is to bias the search so that fast boot
times are the common case.)
SRCU_MAX_NODELAY_PHASE 100 becomes slower.
8. 5.19-rc1 + fix6: fix4 + SRCU_MAX_NODELAY_PHASE 1000000
real 0m11.154s ~12s
user 0m2.919s
sys 0m1.064s
9. 5.19-rc1 + fix7: fix4 + SRCU_MAX_NODELAY_PHASE 10000
real 0m11.258s
user 0m3.113s
sys 0m1.073s
10. 5.19-rc1 + fix8: fix4 + SRCU_MAX_NODELAY_PHASE 100
real 0m30.053s ~ 32s
user 0m2.827s
sys 0m1.161s
By the way, if build kernel on the board in the meantime (using
memory), time become much longer.
real 1m2.763s
11. 5.19-rc1 + fix9: fix4 + SRCU_MAX_NODELAY_PHASE 1000
real 0m11.443s
user 0m3.022s
sys 0m1.052s
This is useful data, thanks! Did you get chance to check between 100
and 1000, to narrow down further, from which point (does need to be
exact value) between 100 and 1000, you start seeing degradation at,
for ex. 250, 500 , ...?
Is it also possible to try experiment 10 and 11 with below diff.
What I have done in below diff is, call srcu_get_delay() only once
in try_check_zero() (and not for every loop iteration); also
retry with a different delay for the extra iteration which is done
when srcu_get_delay(ssp) returns 0.
Once we have this data, can you also try by changing
SRCU_RETRY_CHECK_LONG_DELAY to 100, on top of below diff.
#define SRCU_RETRY_CHECK_LONG_DELAY 100
-------------------------------------------------------------------------
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 6a354368ac1d..3aff2f3e99ab 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -620,6 +620,7 @@ EXPORT_SYMBOL_GPL(__srcu_read_unlock);
* we repeatedly block for 1-millisecond time periods.
*/
#define SRCU_RETRY_CHECK_DELAY 5
+#define SRCU_RETRY_CHECK_LONG_DELAY 5
/*
* Start an SRCU grace period.
@@ -927,12 +928,17 @@ static void srcu_funnel_gp_start(struct
srcu_struct *ssp, struct srcu_data *sdp,
*/
static bool try_check_zero(struct srcu_struct *ssp, int idx, int
trycount)
{
+ unsigned long curdelay;
+ curdelay = !srcu_get_delay(ssp);
for (;;) {
if (srcu_readers_active_idx_check(ssp, idx))
return true;
- if (--trycount + !srcu_get_delay(ssp) <= 0)
+ if (--trycount + curdelay <= 0)
return false;
- udelay(SRCU_RETRY_CHECK_DELAY);
+ if (trycount)
+ udelay(SRCU_RETRY_CHECK_DELAY);
+ else
+ udelay(SRCU_RETRY_CHECK_LONG_DELAY);
}
}
11. 5.19-rc1 + fix9: fix4 + SRCU_MAX_NODELAY_PHASE 1000
real 0m11.443
s user 0m3.022
s sys 0m1.052s
fix10: fix4 + SRCU_MAX_NODELAY_PHASE 500
real 0m11.401s
user 0m2.798s
sys 0m1.328s
fix11: fix4 + SRCU_MAX_NODELAY_PHASE 250
real 0m15.748s
user 0m2.781s
sys 0m1.294s
fix12: fix4 + SRCU_MAX_NODELAY_PHASE 200
real 0m20.704s 21
user 0m2.954s
sys 0m1.226s
fix13: fix4 + SRCU_MAX_NODELAY_PHASE 150
real 0m25.151s
user 0m2.980s
sys 0m1.256s
fix8: fix4 + SRCU_MAX_NODELAY_PHASE 100
real 0m30.053s ~ 32s
user 0m2.827s
sys 0m1.161s
fix14: fix4 + SRCU_MAX_NODELAY_PHASE 100 + SRCU_RETRY_CHECK_LONG_DELAY 5
real 0m19.263s
user 0m3.018s
sys 0m1.211s
fix15: fix4 + SRCU_MAX_NODELAY_PHASE 100 +
SRCU_RETRY_CHECK_LONG_DELAY 100
real 0m9.347s
user 0m3.132s
sys 0m1.041s
Thanks. From the data and experiments done, looks to me that we get
comparable (to 5.18-rc4 ) timings, when we retry without sleep for
time duration close to 4-5 ms, which could be closer to the configured
HZ (as it is 250)? Is it possible to try below configuration on top
of fix15?
If possible can you try with both HZ_1000 and HZ_250?
As multiple fixes are getting combined in experiments, for clarity,
please also share the diff of srcutree.c (on top of baseline) for all
experiments.
16. fix15 + SRCU_MAX_NODELAY_PHASE 20 (10 try_check_zero() calls) +
(long delay scaled to 1 jiffy)
#define SRCU_MAX_NODELAY_TRY_CHECK_PHASE 10
#define SRCU_MAX_NODELAY_PHASE (SRCU_MAX_NODELAY_TRY_CHECK_PHASE * 2)
#define SRCU_RETRY_CHECK_LONG_DELAY \
(USEC_PER_SEC / HZ / SRCU_MAX_NODELAY_TRY_CHECK_PHASE)
17. fix15 + SRCU_MAX_NODELAY_PHASE 20 (10 try_check_zero() calls) +
(long delay scaled to 2 jiffy)
#define SRCU_RETRY_CHECK_LONG_DELAY \
(2 * USEC_PER_SEC / HZ / SRCU_MAX_NODELAY_TRY_CHECK_PHASE)
18. fix15 + SRCU_MAX_NODELAY_PHASE 20 (10 try_check_zero() calls) +
(long delay scaled to 1/2 jiffy)
#define SRCU_RETRY_CHECK_LONG_DELAY \
(USEC_PER_SEC / HZ / SRCU_MAX_NODELAY_TRY_CHECK_PHASE / 2)
fix16: fix15 + SRCU_MAX_NODELAY_PHASE 20 (10 try_check_zero()
calls) + (long delay scaled to 1 jiffy)
real 0m10.120s
user 0m3.885s
sys 0m1.040s
fix17: fix15 + SRCU_MAX_NODELAY_PHASE 20 (10 try_check_zero()
calls) + (long delay scaled to 2 jiffy)
real 0m9.851s
user 0m3.886s
sys 0m1.011s
fix18: fix15 + SRCU_MAX_NODELAY_PHASE 20 (10 try_check_zero()
calls) + (long delay scaled to 1/2 jiffy)
real 0m9.741s
user 0m3.837s
sys 0m1.060s
code push to
https://github.com/Linaro/linux-kernel-uadk/tree/uacce-devel-5.19-srcu-test