netdev - Re: [PATCH v6 08/15] net: add helper executing custom callback from napi

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Message-ID: <9d2c123d-9e1e-4365-a047-e4fe84444ab9@gmail.com>
Date: Mon, 21 Oct 2024 18:16:16 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Paolo Abeni <pabeni@...hat.com>, David Wei <dw@...idwei.uk>,
 io-uring@...r.kernel.org, netdev@...r.kernel.org
Cc: Jens Axboe <axboe@...nel.dk>, Jakub Kicinski <kuba@...nel.org>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jesper Dangaard Brouer <hawk@...nel.org>, David Ahern <dsahern@...nel.org>,
 Mina Almasry <almasrymina@...gle.com>,
 Stanislav Fomichev <stfomichev@...il.com>, Joe Damato <jdamato@...tly.com>,
 Pedro Tammela <pctammela@...atatu.com>
Subject: Re: [PATCH v6 08/15] net: add helper executing custom callback from
 napi

On 10/21/24 15:25, Paolo Abeni wrote:
> Hi,
> 
> On 10/16/24 20:52, David Wei wrote:
>> @@ -6503,6 +6511,41 @@ void napi_busy_loop(unsigned int napi_id,
>>   }
>>   EXPORT_SYMBOL(napi_busy_loop);
>>   
>> +void napi_execute(unsigned napi_id,
>> +		  void (*cb)(void *), void *cb_arg)
>> +{
>> +	struct napi_struct *napi;
>> +	void *have_poll_lock = NULL;
> 
> Minor nit: please respect the reverse x-mas tree order.
> 
>> +
>> +	guard(rcu)();
> 
> Since this will land into net core code, please use the explicit RCU
> read lock/unlock:
> 
> https://elixir.bootlin.com/linux/v6.12-rc3/source/Documentation/process/maintainer-netdev.rst#L387

I missed the doc update, will change it, thanks

>> +	napi = napi_by_id(napi_id);
>> +	if (!napi)
>> +		return;
>> +
>> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
>> +		preempt_disable();
>> +
>> +	for (;;) {
>> +		local_bh_disable();
>> +
>> +		if (napi_state_start_busy_polling(napi, 0)) {
>> +			have_poll_lock = netpoll_poll_lock(napi);
>> +			cb(cb_arg);
>> +			local_bh_enable();
>> +			busy_poll_stop(napi, have_poll_lock, 0, 1);
>> +			break;
>> +		}
>> +
>> +		local_bh_enable();
>> +		if (unlikely(need_resched()))
>> +			break;
>> +		cpu_relax();
> 
> Don't you need a 'loop_end' condition here?

As you mentioned in 14/15, it can indeed spin for long and is bound only
by need_resched(). Do you think it's reasonable to wait for it without a
time limit with NAPI_STATE_PREFER_BUSY_POLL? softirq should yield napi
after it exhausts the budget, it should limit it well enough, what do
you think?

The only ugly part is that I don't want it to mess with the
NAPI_F_PREFER_BUSY_POLL in busy_poll_stop()

busy_poll_stop() {
	...
	clear_bit(NAPI_STATE_IN_BUSY_POLL, &napi->state);
	if (flags & NAPI_F_PREFER_BUSY_POLL) {
		napi->defer_hard_irqs_count = READ_ONCE(napi->dev->napi_defer_hard_irqs);
		timeout = READ_ONCE(napi->dev->gro_flush_timeout);
		if (napi->defer_hard_irqs_count && timeout) {
			hrtimer_start(&napi->timer, ns_to_ktime(timeout), HRTIMER_MODE_REL_PINNED);
			skip_schedule = true;
		}
	}
}

Is it fine to set PREFER_BUSY_POLL but do the stop call without? E.g.

set_bit(NAPI_STATE_PREFER_BUSY_POLL, &napi->state);
...
busy_poll_stop(napi, flags=0);

-- 
Pavel Begunkov