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

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Message-ID: <87r2l1sby0.fsf@linux.intel.com>
Date:   Wed, 20 Jun 2018 08:40:55 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        y2038@...ts.linaro.org, Andi Kleen <andi.kleen@...el.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Jeff Layton <jlayton@...hat.com>, Jan Kara <jack@...e.cz>,
        Brian Foster <bfoster@...hat.com>,
        Deepa Dinamani <deepa.kernel@...il.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Jens Axboe <axboe@...nel.dk>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vfs: replace current_kernel_time64 with ktime equivalent

Arnd Bergmann <arnd@...db.de> writes:
>
> I traced the original addition of the current_kernel_time() call to set
> the nanosecond fields back to linux-2.5.48, where Andi Kleen added a
> patch with subject "nanosecond stat timefields". This adds the original
> call to current_kernel_time and the truncation to the resolution of the
> file system, but makes no mention of the intended accuracy.  At the time,
> we had a do_gettimeofday() interface that on some architectures could
> return a microsecond-resolution timestamp, but there was no interface
> for getting an accurate timestamp in nanosecond resolution, neither inside
> the kernel nor from user space. This makes me suspect that the use of
> coarse timestamps was never really a conscious decision but instead
> a result of whatever API was available 16 years ago.

Kind of. VFS/system calls are expensive enough that you need multiple us
in and out so us resolution was considered good enough.

Also if you do this change you really need to do some benchmarks,
especially on setups without lazy atime. This might potentially
cause a lot more inode flushes.

-Andi