linux-kernel - Re: [RFC net-next 4/5] eventpoll: Trigger napi_busy_loop, if prefer_busy

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Message-ID: <ZrpKaPcAow7vvClC@LQ3V64L9R2>
Date: Mon, 12 Aug 2024 18:46:16 +0100
From: Joe Damato <jdamato@...tly.com>
To: Christoph Hellwig <hch@...radead.org>
Cc: netdev@...r.kernel.org, mkarsten@...terloo.ca,
	amritha.nambiar@...el.com, sridhar.samudrala@...el.com,
	sdf@...ichev.me, Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	"open list:FILESYSTEMS (VFS and infrastructure)" <linux-fsdevel@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC net-next 4/5] eventpoll: Trigger napi_busy_loop, if
 prefer_busy_poll is set

On Mon, Aug 12, 2024 at 06:19:35AM -0700, Christoph Hellwig wrote:
> On Mon, Aug 12, 2024 at 12:57:07PM +0000, Joe Damato wrote:
> > From: Martin Karsten <mkarsten@...terloo.ca>
> > 
> > Setting prefer_busy_poll now leads to an effectively nonblocking
> > iteration though napi_busy_loop, even when busy_poll_usecs is 0.
> 
> Hardcoding calls to the networking code from VFS code seems like
> a bad idea.   Not that I disagree with the concept of disabling
> interrupts during busy polling, but this needs a proper abstraction
> through file_operations.

Thanks for the feedback; the code modified in the this patch was
already calling directly into the networking stack; we didn't add
that call. We added a check on another member of the eventpoll
structure, though.

In general: it may be appropriate for a better abstraction to exist
between fs/eventpoll.c and the networking stack as there are already
many calls into the networking stack from this code.

However, I think that is a much larger change that is not directly
related to what we're proposing, which is: a mechanism for more
efficient epoll-based busy poll which shows significant performance
improvements.

- Joe