linux-kernel - Re: [PATCH v3] trace: Use 64-bit timekeeping

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Date:	Tue, 27 Jan 2015 11:16:06 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Tina Ruchandani <ruchandani.tina@...il.com>
Cc:	Linux Kernel List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v3] trace: Use 64-bit timekeeping

On Tue, 27 Jan 2015 21:25:38 +0530
Tina Ruchandani <ruchandani.tina@...il.com> wrote:

> The ring_buffer_producer uses 'struct timeval' to measure
> its start and end times. 'struct timeval' on 32-bit systems
> will have its tv_sec value overflow in year 2038 and beyond.
> This patch replaces struct timeval with 'ktime_t' which uses
> 64-bit representation for seconds

Doesn't ktime_t actually use 64-bit representation for nanoseconds?

> 
> Suggested-by: Arnd Bergmann <arnd@...db.de>
> Signed-off-by: Tina Ruchandani <ruchandani.tina@...il.com>
> ---
> Changes in v3:
> 	- Use a more efficient way to compute condition for exiting
> 	  loop.
> 	- Fix variable naming - all caps is only for macros.
> Changes in v2:
> 	- Use ktime_t instead of timespec64 for efficiency reasons.
> ---
>  kernel/trace/ring_buffer_benchmark.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/trace/ring_buffer_benchmark.c b/kernel/trace/ring_buffer_benchmark.c
> index 3f9e328..0a50abb 100644
> --- a/kernel/trace/ring_buffer_benchmark.c
> +++ b/kernel/trace/ring_buffer_benchmark.c
> @@ -7,7 +7,7 @@
>  #include <linux/completion.h>
>  #include <linux/kthread.h>
>  #include <linux/module.h>
> -#include <linux/time.h>
> +#include <linux/ktime.h>
>  #include <asm/local.h>
>  
>  struct rb_page {
> @@ -212,8 +212,7 @@ static void ring_buffer_consumer(void)
>  
>  static void ring_buffer_producer(void)
>  {
> -	struct timeval start_tv;
> -	struct timeval end_tv;
> +	ktime_t start_time, end_time, timeout;
>  	unsigned long long time;
>  	unsigned long long entries;
>  	unsigned long long overruns;
> @@ -227,7 +226,9 @@ static void ring_buffer_producer(void)
>  	 * make the system stall)
>  	 */
>  	trace_printk("Starting ring buffer hammer\n");
> -	do_gettimeofday(&start_tv);
> +	start_time = ktime_get();
> +	timeout = ktime_add_ns(start_time,
> +			       ((long long) RUN_TIME) * NSEC_PER_SEC);

Instead of the (long long) typecast, could you define RUN_TIME as 10ULL.

Makes the code a bit cleaner, and we can put that on one line.

Rest looks fine.

-- Steve

>  	do {
>  		struct ring_buffer_event *event;
>  		int *entry;
> @@ -244,7 +245,7 @@ static void ring_buffer_producer(void)
>  				ring_buffer_unlock_commit(buffer, event);
>  			}
>  		}
> -		do_gettimeofday(&end_tv);
> +		end_time = ktime_get();
>  
>  		cnt++;
>  		if (consumer && !(cnt % wakeup_interval))
> @@ -264,7 +265,7 @@ static void ring_buffer_producer(void)
>  			cond_resched();
>  #endif
>  
> -	} while (end_tv.tv_sec < (start_tv.tv_sec + RUN_TIME) && !kill_test);
> +	} while (ktime_before(end_time, timeout) && !kill_test);
>  	trace_printk("End ring buffer hammer\n");
>  
>  	if (consumer) {
> @@ -280,9 +281,7 @@ static void ring_buffer_producer(void)
>  		wait_for_completion(&read_done);
>  	}
>  
> -	time = end_tv.tv_sec - start_tv.tv_sec;
> -	time *= USEC_PER_SEC;
> -	time += (long long)((long)end_tv.tv_usec - (long)start_tv.tv_usec);
> +	time = ktime_us_delta(end_time, start_time);
>  
>  	entries = ring_buffer_entries(buffer);
>  	overruns = ring_buffer_overruns(buffer);

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