Re: [PATCH 3/6] service: reduce average case service core overhead

["Mattias Rönnblom" <mattias.ronnblom@ericsson.com>]

On 2022-10-03 15:33, Van Haaren, Harry wrote:
>> -----Original Message-----
>> From: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
>> Sent: Tuesday, September 6, 2022 5:14 PM
>> To: Van; Haaren; Van Haaren, Harry <harry.van.haaren@intel.com>
>> Cc: dev@dpdk.org; Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>;
>> Morten Brørup <mb@smartsharesystems.com>; nd <nd@arm.com>;
>> mattias.ronnblom <mattias.ronnblom@ericsson.com>
>> Subject: [PATCH 3/6] service: reduce average case service core overhead
>>
>> Optimize service loop so that the starting point is the lowest-indexed
>> service mapped to the lcore in question, and terminate the loop at the
>> highest-indexed service.
>>
>> While the worst case latency remains the same, this patch
>> significantly reduces the service framework overhead for the average
>> case. In particular, scenarios where an lcore only runs a single
>> service, or multiple services which id values are close (e.g., three
>> services with ids 17, 18 and 22), show significant improvements.
>>
>> The worse case is a where the lcore two services mapped to it; one
>> with service id 0 and the other with id 63.
> 
> I like the optimization - nice work. There is one caveat, that with the
> builtin_ctz() call, RTE_SERVICE_NUM_MAX *must* be 64 or lower.
> Today it is defined as 64, but we must ensure that this value cannot
> be changed "by accident" without explicit compilation failures and a
> comment explaining that fact.
>  > There are likely options around making it runtime-dynamic, but I don't
> think the complexity is justified: suggest we use compile-time check
> BUILD_BUG_ON() and error if its > 64?
> 

Sounds like a good idea. The limitations is not new though; the use of 
an uint64_t-based bitmask limits the services to 64 already.

> Note in rte_service_component_register(), we *re-use* IDs when they
> become available, so we can have up to 64 active services at a time, but
> the can register/unregister more times than that. This is a very unlikely
> usage of the services API to continually register-unregister services.
> 
> With the BUILD_BUG_ON() around the 64 MAX value with a comment:
> Acked-by: Harry van Haaren <harry.van.haaren@intel.com>
> 
Thanks for your reviews Harry.

> 
>> On a service lcore serving a single service, the service loop overhead
>> is reduced from ~190 core clock cycles to ~46. (On an Intel Cascade
>> Lake generation Xeon.) On weakly ordered CPUs, the gain is larger,
>> since the loop included load-acquire atomic operations.
>>
>> Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
>> ---
>>   lib/eal/common/rte_service.c | 14 ++++++++++----
>>   1 file changed, 10 insertions(+), 4 deletions(-)
>>
>> diff --git a/lib/eal/common/rte_service.c b/lib/eal/common/rte_service.c
>> index 87df04e3ac..4cac866792 100644
>> --- a/lib/eal/common/rte_service.c
>> +++ b/lib/eal/common/rte_service.c
>> @@ -464,7 +464,6 @@ static int32_t
>>   service_runner_func(void *arg)
>>   {
>>   	RTE_SET_USED(arg);
>> -	uint32_t i;
>>   	const int lcore = rte_lcore_id();
>>   	struct core_state *cs = &lcore_states[lcore];
>>
>> @@ -478,10 +477,17 @@ service_runner_func(void *arg)
>>   			RUNSTATE_RUNNING) {
>>
>>   		const uint64_t service_mask = cs->service_mask;
>> +		uint8_t start_id;
>> +		uint8_t end_id;
>> +		uint8_t i;
>>
>> -		for (i = 0; i < RTE_SERVICE_NUM_MAX; i++) {
>> -			if (!service_registered(i))
>> -				continue;
>> +		if (service_mask == 0)
>> +			continue;
>> +
>> +		start_id = __builtin_ctzl(service_mask);
>> +		end_id = 64 - __builtin_clzl(service_mask);
>> +
>> +		for (i = start_id; i < end_id; i++) {
>>   			/* return value ignored as no change to code flow */
>>   			service_run(i, cs, service_mask, service_get(i), 1);
>>   		}
>