linux-kernel - Re: [PATCH v3 1/2] epoll: add nsec timeout support with epoll

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Date:   Thu, 19 Nov 2020 14:31:31 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     David Laight <David.Laight@...lab.com>,
        Arnd Bergmann <arnd@...nel.org>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Soheil Hassas Yeganeh <soheil.kdev@...il.com>,
        Arnd Bergmann <arnd@...db.de>, Shuo Chen <shuochen@...gle.com>,
        linux-man <linux-man@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH v3 1/2] epoll: add nsec timeout support with epoll_pwait2

On Thu, Nov 19, 2020 at 09:19:35AM -0500, Willem de Bruijn wrote:
> But for epoll, this is inefficient: in ep_set_mstimeout it calls
> ktime_get_ts64 to convert timeout to an offset from current time, only
> to pass it to select_estimate_accuracy to then perform another
> ktime_get_ts64 and subtract this to get back to (approx.) the original
> timeout.
> 
> How about a separate patch that adds epoll_estimate_accuracy with
> the same rules (wrt rt_task, current->timer_slack, nice and upper bound)
> but taking an s64 timeout.
> 
> One variation, since it is approximate, I suppose we could even replace
> division by a right shift?
> 
> After that, using s64 everywhere is indeed much simpler. And with that
> I will revise the new epoll_pwait2 interface to take a long long
> instead of struct timespec.

I think the userspace interface should take a struct timespec
for consistency with ppoll and pselect.  And epoll should use
poll_select_set_timeout() to convert the relative timeout to an absolute
endtime.  Make epoll more consistent with select/poll, not less ...