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Message-Id: <20091216134252.868ea5bf.akpm@linux-foundation.org>
Date: Wed, 16 Dec 2009 13:42:52 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: minyard@....org
Cc: Linux Kernel <linux-kernel@...r.kernel.org>,
Martin Wilck <martin.wilck@...fujitsu.com>,
Jean Delvare <jdelvare@...e.de>,
OpenIPMI Developers <openipmi-developer@...ts.sourceforge.net>
Subject: Re: [PATCH] IPMI: Add parameter to limit CPU usage in kipmid
On Wed, 16 Dec 2009 15:23:54 -0600
Corey Minyard <minyard@....org> wrote:
> From: Martin Wilck <martin.wilck@...fujitsu.com>
>
> In some cases kipmid can use a lot of CPU.
Why is that? Without this information it is hard for others to suggest
alternative implementations.
> This adds a way to tune
> the CPU used by kipmid to help in those cases. By setting
> kipmid_max_busy_us to a value between 100 and 500, it is possible to
> bring down kipmid CPU load to practically 0 without loosing too much
> ipmi throughput performance. Not setting the value, or setting the
> value to zero, operation is unaffected.
Requiring the addition of a module parameter is regrettable. It'd be
better if the code "just works".
> Signed-off-by: Martin Wilck <martin.wilck@...fujitsu.com>
> Cc: Jean Delvare <jdelvare@...e.de>
> Signed-off-by: Corey Minyard <cminyard@...sta.com>
>
> --- linux-2.6.29.4/drivers/char/ipmi/ipmi_si_intf.c 2009-05-19 01:52:34.000000000 +0200
> +++ linux-2.6.29-rc8/drivers/char/ipmi/ipmi_si_intf.c 2009-06-04 15:30:34.855398091 +0200
> @@ -297,6 +297,9 @@
> static int force_kipmid[SI_MAX_PARMS];
> static int num_force_kipmid;
>
> +static unsigned int kipmid_max_busy_us[SI_MAX_PARMS];
> +static int num_max_busy_us;
> +
> static int unload_when_empty = 1;
>
> static int try_smi_init(struct smi_info *smi);
> @@ -927,23 +930,56 @@
> }
> }
>
> +#define ipmi_si_set_not_busy(timespec) \
> + do { (timespec)->tv_nsec = -1; } while (0)
> +#define ipmi_si_is_busy(timespec) ((timespec)->tv_nsec != -1)
These could have been implemented in C. It's better that way.
> +static int ipmi_thread_busy_wait(enum si_sm_result smi_result,
> + const struct smi_info *smi_info,
> + struct timespec *busy_until)
> +{
> + unsigned int max_busy_us = 0;
> +
> + if (smi_info->intf_num < num_max_busy_us)
> + max_busy_us = kipmid_max_busy_us[smi_info->intf_num];
> + if (max_busy_us == 0 || smi_result != SI_SM_CALL_WITH_DELAY)
> + ipmi_si_set_not_busy(busy_until);
> + else if (!ipmi_si_is_busy(busy_until)) {
> + getnstimeofday(busy_until);
> + timespec_add_ns(busy_until, max_busy_us*NSEC_PER_USEC);
> + } else {
> + struct timespec now;
> + getnstimeofday(&now);
> + if (unlikely(timespec_compare(&now, busy_until) > 0)) {
> + ipmi_si_set_not_busy(busy_until);
> + return 0;
> + }
> + }
> + return 1;
> +}
This function would benefit from some documentation.
It's a bit opaque. The setting of timespec.tv_nsec to -1 appears to
have some magical meaning, but it's left to the reader to work out what
that meaning is.
It might be clearer if the return type was `bool', ditto local variable
`busy_wait', below.
> static int ipmi_thread(void *data)
> {
> struct smi_info *smi_info = data;
> unsigned long flags;
> enum si_sm_result smi_result;
> + struct timespec busy_until;
>
> + ipmi_si_set_not_busy(&busy_until);
> set_user_nice(current, 19);
> while (!kthread_should_stop()) {
> + int busy_wait;
> spin_lock_irqsave(&(smi_info->si_lock), flags);
> smi_result = smi_event_handler(smi_info, 0);
> spin_unlock_irqrestore(&(smi_info->si_lock), flags);
> + busy_wait = ipmi_thread_busy_wait(smi_result, smi_info,
> + &busy_until);
> if (smi_result == SI_SM_CALL_WITHOUT_DELAY)
> ; /* do nothing */
> - else if (smi_result == SI_SM_CALL_WITH_DELAY)
> + else if (smi_result == SI_SM_CALL_WITH_DELAY && busy_wait)
> schedule();
> else
> - schedule_timeout_interruptible(1);
> + schedule_timeout_interruptible(0);
> }
> return 0;
> }
hm, what does schedule_timeout(0) do? It sets the timer to go off at
`jiffies' which I suppose means that the timer implementation will run
the callback at the next tick.
If there _is_ a tick. What does it do on NOHZ kernels?
The behaviour depends on HZ (it always did). Has it been tested to
check that performance is acceptable with HZ=100?
Again, it's too hard (IMO) to work out why the code sometimes calls
schedule() and sometimes calls schedule_timeout(0). It's a design
decision which is best communicated with a comment, please.
> @@ -1213,6 +1249,11 @@
> MODULE_PARM_DESC(unload_when_empty, "Unload the module if no interfaces are"
> " specified or found, default is 1. Setting to 0"
> " is useful for hot add of devices using hotmod.");
> +module_param_array(kipmid_max_busy_us, uint, &num_max_busy_us, 0644);
> +MODULE_PARM_DESC(kipmid_max_busy_us,
> + "Max time (in microseconds) to busy-wait for IPMI data before"
> + " sleeping. 0 (default) means to wait forever. Set to 100-500"
> + " if kipmid is using up a lot of CPU time.");
>
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