Re: [dpdk-dev] [PATCH v4 2/2] timer: support EAL functions on Windows

[Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>]

Getting extremely volatile results from get_tsc_hz(). Here's what I get from
10 successive calls:

freq=2660000000
freq=3220000000
freq=3740000000
freq=3870000000
freq=3830000000
freq=4060000000
freq=3370000000
freq=3820000000
freq=7370000000
freq=4970000000

Please see the last comment for the reason (and other comments inline).

On 2020-05-06 15:01 GMT+0300 Fady Bader wrote:
> Implemented the needed Windows eal timer functions.
> 
> Signed-off-by: Fady Bader <fady@mellanox.com>
> ---
[snip]
> diff --git a/lib/librte_eal/windows/eal_timer.c b/lib/librte_eal/windows/eal_timer.c
> new file mode 100644
> index 000000000..108f66b54
> --- /dev/null
> +++ b/lib/librte_eal/windows/eal_timer.c
> @@ -0,0 +1,96 @@
> +/* SPDX-License-Identifier: BSD-3-Clause
> + * Copyright 2020 Mellanox Technologies, Ltd
> + */
> +#include <inttypes.h>
> +#include <time.h>
> +
> +#include <rte_windows.h>
> +#include <rte_common.h>
> +#include <rte_log.h>
> +#include <rte_cycles.h>
> +#include <rte_eal.h>
> +#include <rte_errno.h>

#include "eal_private.h", without it:

../../../lib/librte_eal/windows/eal_timer.c: In function ‘rte_eal_timer_init’:
../../../lib/librte_eal/windows/eal_timer.c:108:2: warning: implicit declaration of function ‘set_tsc_freq’; did you mean ‘get_tsc_freq’? [-Wimplicit-function-declaration]
  108 |  set_tsc_freq();

> +
> +#define US_PER_SEC 1E6
> +#define CYC_PER_10MHZ 1E7
> +
> +/* The frequency of the RDTSC timer resolution */
> +static uint64_t eal_tsc_resolution_hz;
> +

At top level:
../../../lib/librte_eal/windows/eal_timer.c:20:17: warning: ‘eal_tsc_resolution_hz’ defined but not used [-Wunused-variable]
   20 | static uint64_t eal_tsc_resolution_hz;

Note: set_tsc_freq() already caches frequency you calculate here.

> +	due_time.QuadPart = -(us * 10);
> +	if (!SetWaitableTimer(timer, &due_time, 0, NULL, NULL, FALSE)) {
> +		CloseHandle(timer);
> +		RTE_LOG_WIN32_ERR("SetWaitableTimer()");

RTE_LOG_WIN32_ERR() uses GetLastError(), the value of which may be overridden
by a successful call to CloseHandle().

> +		rte_errno = EINVAL;
> +		return;
> +	}
> +	/* start wait for timer for us microseconds */
> +	WaitForSingleObject(timer, INFINITE);
> +	CloseHandle(timer);
> +}
> +
> +uint64_t
> +get_tsc_freq(void)
> +{
> +	LARGE_INTEGER t_start, t_end, elapsed_us;
> +	LARGE_INTEGER frequency;
> +	uint64_t tsc_hz;
> +
> +	if (QueryPerformanceFrequency(&frequency) == 0) {
> +		RTE_LOG_WIN32_ERR("QueryPerformanceFrequency()");
> +		return 0;
> +	}
> +
> +	if (QueryPerformanceCounter(&t_start) != 0) {

Per MSDN, QueryPerformanceFrequency() and QueryPerformanceCounter() always
succeed on all supported Windows versions.

> +		uint64_t end, start = rte_get_tsc_cycles();
> +
> +		rte_delay_us(US_PER_SEC / 10); /* 1/10 second */

Did you mean rte_delay_us_sleep()? rte_delay_us() assumes TSC frequency is
already available. With this fix, results become stable.

-- 
Dmitry Kozlyuk