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

[Samuel Holland]

On 10/9/22 18:45, Palmer Dabbelt wrote:
> 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.

I opened an issue against the SBI spec about what exactly it requires,
but I got no responses:

https://github.com/riscv-non-isa/riscv-sbi-doc/issues/98

My interpretation of the SBI specification is that it does not require
maintaining any hart-local state across a non-retentive hart suspend.
Unless the SBI spec says the timer must fire during/after suspend, then
there is no firmware bug.

> 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.

And if we cannot rely on the interrupt being delivered, we cannot rely
on the SBI time extension to work across cpuidle entry.

> 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.

So by extension, non-retentive cpuidle states cannot be used if the SBI
timer is the only available timer, since there is no hardware broadcast
timer to use as a backup.

> 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?

The motivating scenario for this patch is the C906, where the MTIMER is
in the same reset domain as the CPU, so the timer state is lost during
non-retentive suspend. Without this patch, if riscv_timer_clockevent is
the current clockevent driver, then the CPU fails to wake up from
suspend. However, this same problem would occur on any CPU where the
timer or interrupt delivery stops working during suspend.

>>>      .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.

I'm confused here. The RCU stall is itself a bug, right? Are you sure
this patch is wrongly masking the stall, or is it possibly just avoiding
some buggy code and not causing a stall in the first place?

Regards,
Samuel

> 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