Re: [Qemu-devel] [PATCH 1/3] log: improve performance of qemu_log and qemu_log_mask if disabled

["Alex Bennée" <alex.bennee@linaro.org>]

Denis V. Lunev <den@openvz.org> writes:

> On 10/15/2015 08:23 PM, Alex Bennée wrote:
>> Denis V. Lunev <den@openvz.org> writes:
>>
>>> The patch is intended to avoid to perform any operation including
>>> calculation of log function arguments when the log is not enabled due to
>>> various reasons.
>>>
>>> Functions qemu_log and qemu_log_mask are replaced with variadic macros.
>>> Unfortunately the code is not C99 compatible and we can not use
>>> portable __VA_ARGS__ way. There are a lot of warnings in the other
>>> places with --std=c99. Thus the only way to achive the result is to use
>>> args.. GCC extension.
>>>
>>> Format checking performed by compiler will not suffer by this patch. It
>>> will be done inside in fprintf arguments checking.
>>>
>>> Signed-off-by: Denis V. Lunev <den@openvz.org>
>>> Signed-off-by: Pavel Butsykin <pbutsykin@virtuozzo.com>
>>> CC: Markus Armbruster <armbru@redhat.com>
>>> CC: Luiz Capitulino <lcapitulino@redhat.com>
>>> CC: Eric Blake <eblake@redhat.com>
>>> CC: Peter Maydell <peter.maydell@linaro.org>
>>> ---
>>>   include/qemu/log.h | 17 ++++++++++++++---
>>>   qemu-log.c         | 21 ---------------------
>>>   2 files changed, 14 insertions(+), 24 deletions(-)
>>>
>>> diff --git a/include/qemu/log.h b/include/qemu/log.h
>>> index f880e66..57b8c96 100644
>>> --- a/include/qemu/log.h
>>> +++ b/include/qemu/log.h
>>> @@ -53,7 +53,13 @@ static inline bool qemu_loglevel_mask(int mask)
>>>   
>>>   /* main logging function
>>>    */
>>> -void GCC_FMT_ATTR(1, 2) qemu_log(const char *fmt, ...);
>>> +#define qemu_log(args...)                   \
>>> +    do {                                    \
>>> +        if (!qemu_log_enabled()) {          \
>>> +            break;                          \
>>> +        }                                   \
>>> +        fprintf(qemu_logfile, args);        \
>>> +    } while (0)
>>>
>> I've had one of Peter's patches in my logging improvements queue for a
>> while although it uses a slightly different form which I prefer:
>>
>> -/* log only if a bit is set on the current loglevel mask
>> +/* log only if a bit is set on the current loglevel mask:
>> + * @mask: bit to check in the mask
>> + * @fmt: printf-style format string
>> + * @args: optional arguments for format string
>>    */
>> -void GCC_FMT_ATTR(2, 3) qemu_log_mask(int mask, const char *fmt, ...);
>> -
>> +#define qemu_log_mask(MASK, FMT, ...)                   \
>> +    do {                                                \
>> +        if (unlikely(qemu_loglevel_mask(MASK))) {       \
>> +            qemu_log(FMT, ## __VA_ARGS__);              \
>> +        }                                               \
>>
>> See the message:
>>
>> qemu-log: Avoid function call for disabled qemu_log_mask logging
>
> as far as I can see that patch is one year old at least
> and my version is slightly better, it does the same for
> qemu_log.

Yeah it has been sitting in my queue for a while as part of a larger
series. I haven't been pushing it because the demand for the other
changes isn't super high.

Yes it would be better to do both qemu_log and qemu_log_mask.

> The difference is that old patch has unlikely() hint and does not
> contain break.

I'm not sure what the break form brings. It's personal preference I
guess but to me it is more natural to wrap things inside the condition
then use break to skip out of a block.

> I can revert the condition and add the hint in a couple
> of minutes if this will increase the chance to get
> both things merged. We should have both functions
> as macros.
>
> Will you accept that?

I'll certainly review the next submission. I also suggest you CC
qemu-trivial once you have collected all your r-b and a-b/s-o-b tags.

>
> Den

-- 
Alex Bennée