Subject: Re: [PATCH] powerpc/pseries: Avoid context switch in EEH reset if required

From: Gavin Shan <[email-masked]>
Date: Wed, 21 Jan 2015 09:56:07 +1100

On Tue, Jan 20, 2015 at 10:28:16AM +0100, Benjamin Herrenschmidt wrote:
>On Mon, 2015-01-19 at 09:47 +1100, Gavin Shan wrote:
>> On pseries platform, the EEH reset backend pseries_eeh_reset() can
>> be called in atomic context as follows. For this case, we should
>> call udelay() instead of msleep() to avoid context switching.
>> 
>>      drivers/scsi/ipr.c::ipr_reset_slot_reset_done()
>>      drivers/pci/pci.c::pci_set_pcie_reset_state()
>>      arch/powerpc/kernel/eeh.c::pcibios_set_pcie_reset_state()
>>      arch/powerpc/platforms/pseries/eeh_pseries.c::pseries_eeh_reset()
>
>It's not acceptable to introduce multi-millisecond delays at interrupt
>time. In fact, we should generally not use udelay in such context.
>
>I understand that this is an exceptional error handling case but it's
>still not right.
>

Yes, I agree it's unsafe to udelay for multi-milliseconds as the queued
works in atomic context is expected to be completed as soon as possible.

>Are there many other users of pci_set_pcie_reset_state() at interrupt
>time ? Can we have a discussion with the PCI folks as to whether that
>should be legal or not ?
>
>I'm tempted to require that it's made illegal.

Currently, there are 2 drivers calling this function: IPR and misc/genwqe.
Also, VFIO would call this function for IBM and Mellanox adapters in PowerKVM
repository. For now, IPR driver is the only one call this function in atomic
context. 

Sure, I'll send one email to confirm with PCI folks. I guess it's illegal
to call pci_set_pcie_reset_state() in atomic context. If it's the case,
I'm afraid Wendy has to change IPR driver to replace the reset timer with
something else (e.g. workqueue).

Thanks,
Gavin

>
>Ben.
>
>> Signed-off-by: Gavin Shan <[email-masked]>
>> Tested-by: Wen Xiong<[email-masked]>
>> ---
>>  arch/powerpc/platforms/pseries/eeh_pseries.c | 12 ++++++++----
>>  1 file changed, 8 insertions(+), 4 deletions(-)
>> 
>> diff --git a/arch/powerpc/platforms/pseries/eeh_pseries.c b/arch/powerpc/platforms/pseries/eeh_pseries.c
>> index a6c7e19..67623a3 100644
>> --- a/arch/powerpc/platforms/pseries/eeh_pseries.c
>> +++ b/arch/powerpc/platforms/pseries/eeh_pseries.c
>> @@ -503,8 +503,7 @@ static int pseries_eeh_get_state(struct eeh_pe *pe, int *state)
>>   */
>>  static int pseries_eeh_reset(struct eeh_pe *pe, int option)
>>  {
>> -	int config_addr;
>> -	int ret;
>> +	int config_addr, delay, ret;
>>  
>>  	/* Figure out PE address */
>>  	config_addr = pe->config_addr;
>> @@ -528,9 +527,14 @@ static int pseries_eeh_reset(struct eeh_pe *pe, int option)
>>  	/* We need reset hold or settlement delay */
>>  	if (option == EEH_RESET_FUNDAMENTAL ||
>>  	    option == EEH_RESET_HOT)
>> -		msleep(EEH_PE_RST_HOLD_TIME);
>> +		delay = EEH_PE_RST_HOLD_TIME;
>> +	else
>> +		delay = EEH_PE_RST_SETTLE_TIME;
>> +
>> +	if (in_atomic())
>> +		udelay(delay * 1000);
>>  	else
>> -		msleep(EEH_PE_RST_SETTLE_TIME);
>> +		msleep(delay);
>>  
>>  	return ret;
>>  }
>
>