[linux-dvb] High CPU load in "top" due to budget_av slot polling

Robert Schedel r.schedel at yahoo.de
Sun Apr 20 21:43:26 CEST 2008


Oliver Endriss wrote:
> Robert Schedel wrote:
>> Oliver Endriss wrote:
>>> Robert Schedel wrote:
>>>>
>>>> Enabling debug traces shows that polling for the PSR in function 
>>>> 'saa7146_wait_for_debi_done_sleep' constantly times out after 250x1ms 
>>>> sleeps:
>>>>
>>>>  > saa7146: saa7146_wait_for_debi_done_sleep(): saa7146 (0): 
>>>> saa7146_wait_for_debi_done_sleep timed out while waiting for transfer 
>>>> completion
>>>>
>>>> Increasing the 250ms did not avoid the timeout. And as I understood from 
>>>> previous list mails, this timeout is normal when no CI/CAM is connected 
>>>> to the DEBI. However, for me the high frequency polling does not make 
>>>> sense if someone does not plan to plug in a CI/CAM.
>>>>
>>>> When commenting out two lines in 'dvb_ca_en50221_thread_update_delay' to 
>>>> increase the polling timer for slotstate NONE from 100ms (!) to 60s, the 
>>>> CPU load went down to 0. So this is some kind of workaround for me.
>>> Afaics the polling interval could be increased to something like 5s or
>>> 10s if (and only if) the slot is empty. Could you provide a patch?
>> Attached a patch for 2.6.24.4. Opinions?
> 
> Basically it should work but it has to be tested with CI/CAM, too.

Correct, unfortunately I cannot test it against a CI.

> Furthermore it is not sufficient to test with budget-av because many
> other drivers will be affected.
> 
> So I would prefer a patch which does not touch behaviour for other card
> drivers (if possible).

To my understanding of the DVB code dvb_ca_en50221 is only referenced by 
budget_av and budget_ci, at least in the vanilla kernel 2.6.25. The 
patch only changes the timer for slot state EMPTY if 
DVB_CA_EN50221_FLAG_IRQ_CAMCHANGE is not set, which is for
1) budget_av
2) budget_ci if the CI firmware version is 0xa2 (because IRQs for CAM 
change are not supported in this version)

And those two cases are probably affected by the load issue and should 
be fixed.

Of course, we could add another DVB_CA_EN50221 flag solely for budget_av 
to exclude case 2), but does this make sense? Who is available to test 
against a budget_ci with FW=0xa2 whether it is affected by the load issue?

> Please note for 'final' patches:
> Always run 'checkpatch.pl' and fix the errors.
> Sorry for that. :-(

Sorry, forgot to call the check script. Attached a revised patch 
(against 2.6.25).

>> Regarding DEBI_E: Just found a av7110 code comment which also reflects
>> what my recent tests showed:
>>           /* Note: Don't try to handle the DEBI error irq (MASK_18), in
>>            * intel mode the timeout is asserted all the time...
>>            */
>>
>> So really only DEBI_S would be left, see below.
> 
> Did you check whether DEBI_S and/or DEBI_E are ever asserted with your
> setup? If not, an interrupt would never occur anyway...

DEBI_E was always asserted (as described in the av7110 code comment), so 
it was worthless. DEBI_S was never asserted without CI (therefore the 
250ms timeout), so it would probably only be received when a CI is used. 
But as described in my other email with measurements, it seems that 
there is no need to optimize the debi_done function further.

>>>> 4. Are the high timeout periods in debi_done (50ms/250ms) in relation to 
>>>> the 100ms poll timer intended? (I found the recent patch to this code in 
>>>> the mailing list end of last year)
>>> That patch was applied to reduce the load on the pci bus in busy-wait
>>> mode. Basically it did not change anything for cam polling. (In fact I
>>> was not aware that the CAM was polled every 100ms. Imho this should be
>>> fixed.)
>> Only wondered whether the 250ms might have been smaller in former driver
>> versions.
> 
> Iirc it should be even worse with older drivers.
> 
> Basically the 250ms timeout is just a last resort to escape from the
> loop, if the debi transfer hangs for some reason. We might try to reduce
> the timeout but I don't know how far we can go. (Touching 'magic' values
> might be dangeous.)

As above, according to my measurements we would not need to change the 
250ms timeout.

Regards,
Robert
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