From: Prarit Bhargava <prarit@redhat.com>
To: John Stultz <john.stultz@linaro.org>, Joel Fernandes <joelaf@google.com>
Cc: lkml <linux-kernel@vger.kernel.org>,
Mark Salyzyn <salyzyn@android.com>,
Jonathan Corbet <corbet@lwn.net>, Petr Mladek <pmladek@suse.com>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Steven Rostedt <rostedt@goodmis.org>,
Thomas Gleixner <tglx@linutronix.de>,
Stephen Boyd <sboyd@codeaurora.org>,
Andrew Morton <akpm@linux-foundation.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Christoffer Dall <cdall@linaro.org>,
Deepa Dinamani <deepa.kernel@gmail.com>,
Ingo Molnar <mingo@kernel.org>,
Joel Fernandes <joelaf@google.com>,
Kees Cook <keescook@chromium.org>,
Peter Zijlstra <peterz@infradead.org>,
Geert Uytterhoeven <geert+renesas@glider.be>,
"Luis R. Rodriguez" <mcgrof@kernel.org>,
Nicholas Piggin <npiggin@gmail.com>,
"Jason A. Donenfeld" <Jason@zx2c4.com>,
Olof Johansson <olof@lixom.net>,
Josh Poimboeuf <jpoimboe@redhat.com>,
linux-doc@vger.kernel.org
Subject: Re: [PATCH 2/2 v8] printk: Add monotonic, boottime, and realtime timestamps
Date: Fri, 25 Aug 2017 10:46:21 -0400 [thread overview]
Message-ID: <dac013d8-e70d-a89a-b2fb-d7651df983b0@redhat.com> (raw)
In-Reply-To: <CALAqxLUd4nEiayhEMCRCmDm4NrU+4T_VkmafRxJepbGQUg-iEQ@mail.gmail.com>
On 08/24/2017 02:50 PM, John Stultz wrote:
> On Thu, Aug 24, 2017 at 6:42 AM, Prarit Bhargava <prarit@redhat.com> wrote:
>> --- a/include/linux/timekeeping.h
>> +++ b/include/linux/timekeeping.h
>> @@ -239,6 +239,7 @@ static inline u64 ktime_get_raw_ns(void)
>> extern u64 ktime_get_mono_fast_ns(void);
>> extern u64 ktime_get_raw_fast_ns(void);
>> extern u64 ktime_get_boot_fast_ns(void);
>> +extern u64 ktime_get_real_offset(void);
>>
>> /*
>> * Timespec interfaces utilizing the ktime based ones
>> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
>> index fc47863f629c..dd972bc5c88b 100644
>> --- a/kernel/printk/printk.c
>> +++ b/kernel/printk/printk.c
> ...
>> + * printk_get_real_ns: - Return a realtime timestamp for printk messages
>> + * On 32-bit systems selecting the real clock printk timestamp may lead to
>> + * unlikely situations where a timestamp is wrong because the real time offset
>> + * is read without the protection of a sequence lock.
>> + */
>> +static u64 printk_get_real_ns(void)
>> +{
>> + return ktime_get_mono_fast_ns() + ktime_get_real_offset();
>> +}
>> +
> ...
>> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
>> index d111039e0245..de07bb5ffef5 100644
>> --- a/kernel/time/timekeeping.c
>> +++ b/kernel/time/timekeeping.c
>> @@ -508,6 +508,11 @@ u64 notrace ktime_get_boot_fast_ns(void)
>> }
>> EXPORT_SYMBOL_GPL(ktime_get_boot_fast_ns);
>>
>> +u64 ktime_get_real_offset(void)
>> +{
>> + return ktime_to_ns(tk_core.timekeeper.offs_real);
>> +}
>> +
>
> Apologies! One last nit here. So if we're going to export
> ktime_get_real_offset(), folks are going to use it, and there is no
> documentation about its very sketchy behavioral limits as an
> interface, except elsewhere in the printk code.
>
> Instead of doing that, could you export a
> __ktime_get_real_fast_ns_unsafe() function, which has its limits
> (calculating the realtime w/o locks, thus may return completely bad
> values occasionally) clearly documented in the timekeeping code?
np. I'm going to copy the code for
u64 notrace ktime_get_boot_fast_ns(void)
but I'm unsure why the function is marked "notrace", and if
__ktime_get_real_fast_ns_unsafe() must be as well? I don't see anything in the
git log that indicates why the function is notrace.
I've added Joel to this thread ...
P.
next prev parent reply other threads:[~2017-08-25 14:46 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-08-24 13:42 [PATCH 0/2 v8] printk: Add new timestamps Prarit Bhargava
2017-08-24 13:42 ` [PATCH 1/2 v8] time: Make fast functions return 0 before timekeeping is initialized Prarit Bhargava
2017-08-24 13:42 ` [PATCH 2/2 v8] printk: Add monotonic, boottime, and realtime timestamps Prarit Bhargava
2017-08-24 18:50 ` John Stultz
2017-08-25 14:46 ` Prarit Bhargava [this message]
2017-08-25 14:58 ` Steven Rostedt
2017-08-25 4:52 ` Sergey Senozhatsky
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