linux-kernel - Re: [PATCH] vfs: replace current_kernel

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Date:   Wed, 20 Jun 2018 09:19:18 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...hat.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        y2038 Mailman List <y2038@...ts.linaro.org>,
        Brian Foster <bfoster@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andi Kleen <andi.kleen@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Deepa Dinamani <deepa.kernel@...il.com>
Subject: Re: [PATCH] vfs: replace current_kernel_time64 with ktime equivalent

Arnd Bergmann <arnd@...db.de> writes:
>
> To clarify: current_kernel_time() uses at most millisecond resolution rather
> than microsecond, as tkr_mono.xtime_nsec only gets updated during the
> timer tick.

Ah you're right. I remember now: the motivation was to make sure there
is basically no overhead. In some setups the full gtod can be rather
slow, particularly if it falls back to some crappy timer.

I think it would be ok if it falls back to jiffies if TSC or a similar
fast timer doesn't work. But the function you're using likely
doesn't do that?

> Has that time scale changed over the past 16 years as CPUs got faster
> (and system call entry times slower down again with recent changes)?

Maybe a bit, but not substantially.



-Andi