netdev - Re: [PATCH bpf-next v3 01/10] net: introduce preferred busy-polling

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Date:   Mon, 23 Nov 2020 16:04:12 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Björn Töpel <bjorn.topel@...il.com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        Björn Töpel 
        <bjorn.topel@...el.com>, magnus.karlsson@...el.com, ast@...nel.org,
        daniel@...earbox.net, maciej.fijalkowski@...el.com,
        sridhar.samudrala@...el.com, jesse.brandeburg@...el.com,
        qi.z.zhang@...el.com, edumazet@...gle.com,
        jonathan.lemon@...il.com, maximmi@...dia.com
Subject: Re: [PATCH bpf-next v3 01/10] net: introduce preferred busy-polling

On Thu, 19 Nov 2020 09:30:15 +0100 Björn Töpel wrote:
> +	/* The NAPI context has more processing work, but busy-polling
> +	 * is preferred. Exit early.
> +	 */
> +	if (napi_prefer_busy_poll(n)) {
> +		if (napi_complete_done(n, work)) {
> +			/* If timeout is not set, we need to make sure
> +			 * that the NAPI is re-scheduled.
> +			 */
> +			napi_schedule(n);
> +		}
> +		goto out_unlock;
> +	}

Do we really need to go through napi_complete_done() here?

Isn't it sufficient to check:

	if (napi_prefer_busy_poll(n) && 
	    hrtimer_active(&n->timer)) // not 100% sure this is the
	                               // right helper for the check

If timer is scheduled it will fire and worst case sirq will kick back
in after timeout. napi_complete_done() should had been called by the
driver already to schedule the timer. If the driver doesn't call
napi_complete_done() we should not allow it to use busy_poll() anyway.