Re: [PATCH] clocksource/drivers/riscv: Events are stopped during CPU suspend

From: Palmer Dabbelt
Date: Sun Oct 09 2022 - 20:11:10 EST

On Thu, 29 Sep 2022 14:50:45 PDT (-0700), Conor Dooley wrote:
On Sun, May 08, 2022 at 08:21:21PM -0500, Samuel Holland wrote:
Some implementations of the SBI time extension depend on hart-local
state (for example, CSRs) that are lost or hardware that is powered
down when a CPU is suspended. To be safe, the clockevents driver
cannot assume that timer IRQs will be received during CPU suspend.

Fixes: 62b019436814 ("clocksource: new RISC-V SBI timer driver")
Signed-off-by: Samuel Holland <samuel@xxxxxxxxxxxx>
---

drivers/clocksource/timer-riscv.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/clocksource/timer-riscv.c b/drivers/clocksource/timer-riscv.c
index 1767f8bf2013..593d5a957b69 100644
--- a/drivers/clocksource/timer-riscv.c
+++ b/drivers/clocksource/timer-riscv.c
@@ -34,7 +34,7 @@ static int riscv_clock_next_event(unsigned long delta,
static unsigned int riscv_clock_event_irq;
static DEFINE_PER_CPU(struct clock_event_device, riscv_clock_event) = {
.name = "riscv_timer_clockevent",
- .features = CLOCK_EVT_FEAT_ONESHOT,
+ .features = CLOCK_EVT_FEAT_ONESHOT | CLOCK_EVT_FEAT_C3STOP,

This is listed as being x86-specific in the header, but there's a hanful of other ports that enable it for timers as well. Looks like arm is setting this based on DT, which seems reasonable to me: we're working around a firmware bug, there should be some way to turn off that workaround for firmware that doesn't have the bug. Looks like Intel already
turns this off when ARAT is supported, which seems to be the case for anything
modern, so maybe we're just tripping up on some untested behavior here? I'm
not sure exactly how we should probe this, but having it only enabled when we
need the workaround seems like the right way to go.

That said, I'm not actually sure this C3STOP feature does what we want given the commit description. The timers on RISC-V are sort of in this odd middle-ground between being per-CPU timers and system timers: the time they produce is global (or at least close, due to the mtime synchronization rules) but the actual interrupts are only one-shot and only local. From poking around the code I think this just tries to setup a periodic broadcast timer, but since we use software fallbacks to emulate those we'll still end up losing the interrupts/ticks if the CPU that was asked for an interrupt has gone to sleep and lost that state.

I'm not sure if I'm just misunderstanding what's going on here, though. Is there something that describes the behavior this fixes in more detail?

.rating = 100,
.set_next_event = riscv_clock_next_event,
};

After a bit of a painful bisection (with a misdirection into the v5.19
printk reverts along the way) I have arrived at this commit for causing
me some issues.

If an AXI read to the PCIe controller on PolarFire SoC times out, the
system will stall, with an expected:
io scheduler mq-deadline registered
io scheduler kyber registered
microchip-pcie 2000000000.pcie: host bridge /soc/pcie@2000000000 ranges:
microchip-pcie 2000000000.pcie: MEM 0x2008000000..0x2087ffffff -> 0x0008000000
microchip-pcie 2000000000.pcie: sec error in pcie2axi buffer
microchip-pcie 2000000000.pcie: ded error in pcie2axi buffer
microchip-pcie 2000000000.pcie: axi read request error
microchip-pcie 2000000000.pcie: axi read timeout
microchip-pcie 2000000000.pcie: sec error in pcie2axi buffer
microchip-pcie 2000000000.pcie: ded error in pcie2axi buffer
microchip-pcie 2000000000.pcie: sec error in pcie2axi buffer
microchip-pcie 2000000000.pcie: ded error in pcie2axi buffer
microchip-pcie 2000000000.pcie: sec error in pcie2axi buffer
microchip-pcie 2000000000.pcie: ded error in pcie2axi buffer
Freeing initrd memory: 7336K
mc_event_handler: 667402 callbacks suppressed
microchip-pcie 2000000000.pcie: sec error in pcie2axi buffer
microchip-pcie 2000000000.pcie: ded error in pcie2axi buffer
microchip-pcie 2000000000.pcie: sec error in pcie2axi buffer
microchip-pcie 2000000000.pcie: ded error in pcie2axi buffer
microchip-pcie 2000000000.pcie: sec error in pcie2axi buffer
microchip-pcie 2000000000.pcie: ded error in pcie2axi buffer
microchip-pcie 2000000000.pcie: sec error in pcie2axi buffer
microchip-pcie 2000000000.pcie: ded error in pcie2axi buffer
microchip-pcie 2000000000.pcie: sec error in pcie2axi buffer
microchip-pcie 2000000000.pcie: ded error in pcie2axi buffer
mc_event_handler: 666588 callbacks suppressed
<truncated>
microchip-pcie 2000000000.pcie: ded error in pcie2axi buffer
mc_event_handler: 666748 callbacks suppressed
microchip-pcie 2000000000.pcie: sec error in pcie2axi buffer
microchip-pcie 2000000000.pcie: ded error in pcie2axi buffer
microchip-pcie 2000000000.pcie: sec error in pcie2axi buffer
microchip-pcie 2000000000.pcie: ded error in pcie2axi buffer
microchip-pcie 2000000000.pcie: sec error in pcie2axi buffer
microchip-pcie 2000000000.pcie: ded error in pcie2axi buffer
microchip-pcie 2000000000.pcie: sec error in pcie2axi buffer
microchip-pcie 2000000000.pcie: ded error in pcie2axi buffer
microchip-pcie 2000000000.pcie: sec error in pcie2axi buffer
microchip-pcie 2000000000.pcie: ded error in pcie2axi buffer
rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
rcu: 0-...0: (1 GPs behind) idle=19f/1/0x4000000000000002 softirq=34/36 fqs=2626
(detected by 1, t=5256 jiffies, g=-1151, q=1143 ncpus=4)
Task dump for CPU 0:
task:swapper/0 state:R running task stack: 0 pid: 1 ppid: 0 flags:0x00000008
Call Trace:
mc_event_handler: 666648 callbacks suppressed

With this patch applied, the system just locks up without RCU stalling:
io scheduler mq-deadline registered
io scheduler kyber registered
microchip-pcie 2000000000.pcie: host bridge /soc/pcie@2000000000 ranges:
microchip-pcie 2000000000.pcie: MEM 0x2008000000..0x2087ffffff -> 0x0008000000
microchip-pcie 2000000000.pcie: sec error in pcie2axi buffer
microchip-pcie 2000000000.pcie: ded error in pcie2axi buffer
microchip-pcie 2000000000.pcie: axi read request error
microchip-pcie 2000000000.pcie: axi read timeout
microchip-pcie 2000000000.pcie: sec error in pcie2axi buffer
microchip-pcie 2000000000.pcie: ded error in pcie2axi buffer
microchip-pcie 2000000000.pcie: sec error in pcie2axi buffer
microchip-pcie 2000000000.pcie: ded error in pcie2axi buffer
microchip-pcie 2000000000.pcie: sec error in pcie2axi buffer
microchip-pcie 2000000000.pcie: ded error in pcie2axi buffer
Freeing initrd memory: 7332K

As of yet, I have no idea if RCU stalls for other reasons would also be
lost.

Sorry this broke stuff. I'm not entirely sure why this would mask RCU stalls, but it seems like we're hitting some pretty odd paths here and I'm not sure this is expected to work at all for us.

If non-x86 architectures are meant to be able to set CLOCK_EVT_FEAT_C3STOP, maybe we should document what it's supposed to do in a more platform-agnostic fashion?

Thanks,
Conor.

git bisect start
# status: waiting for both good and bad commits
# good: [7699f7aacf3ebfee51c670b6f796b2797f0f7487] RISC-V: Prepare dropping week attribute from arch_kexec_apply_relocations[_add]
git bisect good 7699f7aacf3ebfee51c670b6f796b2797f0f7487
# bad: [63d5172e148bcc174398040861d867bbd2770be4] HACK: jogness
git bisect bad 63d5172e148bcc174398040861d867bbd2770be4
# good: [2518f226c60d8e04d18ba4295500a5b0b8ac7659] Merge tag 'drm-next-2022-05-25' of git://anongit.freedesktop.org/drm/drm
git bisect good 2518f226c60d8e04d18ba4295500a5b0b8ac7659
# good: [907bb57aa7b471872aab2f2e83e9713a145673f9] Merge tag 'pinctrl-v5.19-1' of git://git.kernel.org/pub/scm/linux/kernel/git/linusw/linux-pinctrl
git bisect good 907bb57aa7b471872aab2f2e83e9713a145673f9
# good: [4ad680f083ec360e0991c453e18a38ed9ae500d7] Merge tag 'staging-5.19-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging
git bisect good 4ad680f083ec360e0991c453e18a38ed9ae500d7
# good: [23df9ba64bb9e26cfee6b34f5c3ece49a8a61ee1] Merge tag 'for-5.19/parisc-2' of git://git.kernel.org/pub/scm/linux/kernel/git/deller/parisc-linux
git bisect good 23df9ba64bb9e26cfee6b34f5c3ece49a8a61ee1
# bad: [7a68065eb9cd194cf03f135c9211eeb2d5c4c0a0] Merge tag 'gpio-fixes-for-v5.19-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/brgl/linux
git bisect bad 7a68065eb9cd194cf03f135c9211eeb2d5c4c0a0
# bad: [1f192b9e8d8a5c619b33a868fb1af063af65ce5d] Merge tag 'drm-misc-fixes-2022-06-09' of git://anongit.freedesktop.org/drm/drm-misc into drm-fixes
git bisect bad 1f192b9e8d8a5c619b33a868fb1af063af65ce5d
# good: [b2c9a83d262a8feb022e24e9f9aadb66cb10a7a8] Merge tag 'scsi-misc' of git://git.kernel.org/pub/scm/linux/kernel/git/jejb/scsi
git bisect good b2c9a83d262a8feb022e24e9f9aadb66cb10a7a8
# bad: [e17fee8976c3d2ccf9add6d6c8912a37b025d840] Merge tag 'mm-nonmm-stable-2022-06-05' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
git bisect bad e17fee8976c3d2ccf9add6d6c8912a37b025d840
# bad: [c049ecc523171481accd2c83f79ffeecbf53a915] Merge tag 'timers-core-2022-06-05' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
git bisect bad c049ecc523171481accd2c83f79ffeecbf53a915
# bad: [9c04a8ff03def4df3f81219ffbe1ec9b44ff5348] clocksource/drivers/oxnas-rps: Fix irq_of_parse_and_map() return value
git bisect bad 9c04a8ff03def4df3f81219ffbe1ec9b44ff5348
# bad: [7160d9c4cce94612d5f42a5db392cd606a38737a] clocksource/drivers/armada-370-xp: Convert to SPDX identifier
git bisect bad 7160d9c4cce94612d5f42a5db392cd606a38737a
# bad: [a98399cbc1e05f7b977419f03905501d566cf54e] clocksource/drivers/sp804: Avoid error on multiple instances
git bisect bad a98399cbc1e05f7b977419f03905501d566cf54e
# good: [41929c9f628b9990d33a200c54bb0c919e089aa8] clocksource/drivers/ixp4xx: Drop boardfile probe path
git bisect good 41929c9f628b9990d33a200c54bb0c919e089aa8
# bad: [232ccac1bd9b5bfe73895f527c08623e7fa0752d] clocksource/drivers/riscv: Events are stopped during CPU suspend
git bisect bad 232ccac1bd9b5bfe73895f527c08623e7fa0752d
# first bad commit: [232ccac1bd9b5bfe73895f527c08623e7fa0752d] clocksource/drivers/riscv: Events are stopped during CPU suspend