netdev - Re: [PATCH net-next] net: avoid irqsave in skb_defer_free

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Date:   Wed, 18 Jan 2023 20:19:54 +0100
From:   Jesper Dangaard Brouer <jbrouer@...hat.com>
To:     Jacob Keller <jacob.e.keller@...el.com>, netdev@...r.kernel.org
Cc:     brouer@...hat.com, Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>, edumazet@...gle.com,
        pabeni@...hat.com
Subject: Re: [PATCH net-next] net: avoid irqsave in skb_defer_free_flush


On 17/01/2023 20.29, Jacob Keller wrote:
> 
> On 1/17/2023 4:29 AM, Jesper Dangaard Brouer wrote:
>> The spin_lock irqsave/restore API variant in skb_defer_free_flush can
>> be replaced with the faster spin_lock irq variant, which doesn't need
>> to read and restore the CPU flags.
>>
>> Using the unconditional irq "disable/enable" API variant is safe,
>> because the skb_defer_free_flush() function is only called during
>> NAPI-RX processing in net_rx_action(), where it is known the IRQs
>> are enabled.
>>
> 
> Did you mean disabled here? If IRQs are enabled that would mean the
> interrupt could be triggered and we would need to irqsave, no?

I do mean 'enabled' in the text here.

As you can see in net_rx_action() we are allowed to perform code like:

	local_irq_disable();
	list_splice_init(&sd->poll_list, &list);
	local_irq_enable();

Disabling local IRQ without saving 'flags' and unconditionally enabling
local IRQs again.  Thus, in skb_defer_free_flush() we can do the same,
without saving 'flags'.  Hope it makes it more clear.


>> Expected gain is 14 cycles from avoiding reading and restoring CPU
>> flags in a spin_lock_irqsave/restore operation, measured via a
>> microbencmark kernel module[1] on CPU E5-1650 v4 @ 3.60GHz.
>>
>> Microbenchmark overhead of spin_lock+unlock:
>>   - spin_lock_unlock_irq     cost: 34 cycles(tsc)  9.486 ns
>>   - spin_lock_unlock_irqsave cost: 48 cycles(tsc) 13.567 ns
>>
> 
> Fairly minor change in perf, and..
> 
>> We don't expect to see a measurable packet performance gain, as
>> skb_defer_free_flush() is called infrequently once per NIC device NAPI
>> bulk cycle and conditionally only if SKBs have been deferred by other
>> CPUs via skb_attempt_defer_free().
>>
> 
> Not really measurable as its not called enough, but..
> 
>> [1] https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/lib/time_bench_sample.c
>>
>> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
>> ---
>>   net/core/dev.c |    5 ++---
>>   1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/net/core/dev.c b/net/core/dev.c
>> index cf78f35bc0b9..9c60190fe352 100644
>> --- a/net/core/dev.c
>> +++ b/net/core/dev.c
>> @@ -6616,17 +6616,16 @@ static int napi_threaded_poll(void *data)
>>   static void skb_defer_free_flush(struct softnet_data *sd)
>>   {
>>   	struct sk_buff *skb, *next;
>> -	unsigned long flags;
>>   
>>   	/* Paired with WRITE_ONCE() in skb_attempt_defer_free() */
>>   	if (!READ_ONCE(sd->defer_list))
>>   		return;
>>   
>> -	spin_lock_irqsave(&sd->defer_lock, flags);
>> +	spin_lock_irq(&sd->defer_lock);
>>   	skb = sd->defer_list;
>>   	sd->defer_list = NULL;
>>   	sd->defer_count = 0;
>> -	spin_unlock_irqrestore(&sd->defer_lock, flags);
>> +	spin_unlock_irq(&sd->defer_lock);
>>   
> 
> It's also less code and makes it more clear what dependency this section
> has.
> 
> Seems ok to me, with the minor nit I think in the commit message:
> 
> Reviewed-by: Jacob Keller <jacob.e.keller@...el.com>

Thanks for the review.
--Jesper