linux-kernel - Re: [PATCH 1/4] ACPI: Add acpi_pr

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Message-ID: <1342654349.3010.83.camel@misato.fc.hp.com>
Date:	Wed, 18 Jul 2012 17:32:29 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Joe Perches <joe@...ches.com>
Cc:	shuah.khan@...com, lenb@...nel.org, linux-acpi@...r.kernel.org,
	linux-kernel@...r.kernel.org, bhelgaas@...gle.com,
	isimatu.yasuaki@...fujitsu.com, liuj97@...il.com,
	srivatsa.bhat@...ux.vnet.ibm.com, prarit@...hat.com,
	imammedo@...hat.com, vijaymohan.pandarathil@...com,
	shuahkhan@...il.com
Subject: Re: [PATCH 1/4] ACPI: Add acpi_pr_<level>() interfaces

On Wed, 2012-07-18 at 15:49 -0700, Joe Perches wrote:
> On Wed, 2012-07-18 at 16:26 -0600, Toshi Kani wrote:
> > On Wed, 2012-07-18 at 15:59 -0600, Shuah Khan wrote:
> > > On Wed, 2012-07-18 at 14:40 -0600, Toshi Kani wrote:
> > > > This patch introduces acpi_pr_<level>(), where <level> is a message
> > > > level such as err/warn/info, to support improved logging messages
> > > > for ACPI, esp. in hotplug operations.
> []
> > > > diff --git a/drivers/acpi/utils.c b/drivers/acpi/utils.c
> []
> > > > @@ -454,3 +454,35 @@ acpi_evaluate_hotplug_ost(acpi_handle handle, u32 source_event,
> []
> > > > +	struct acpi_buffer buffer = {ACPI_ALLOCATE_BUFFER};
> []
> > > > +	ret = acpi_get_name(handle, ACPI_FULL_PATHNAME, &buffer);
> > > 
> > > One big problem I see with this approach is now each acpi_printk() will
> > > result in a call to acpi_get_name() which will invoke several ACPI
> > > calls, including a call to acpi_ut_initialize_buffer() which allocates
> > > buffer. Is this really warranted? What is the performance impact of this
> > > change?
> []
> > This interface is intended to be used by acpi_pr_<level>(), which is
> > used for error, warning, debugging, etc.  It is not intended to be used
> > in any performance path.
> 
> While it's not performance critical, perhaps the buffer
> alloc/free could be avoided by using stack. Something like:
> 
> 	char name[ACPI_PATH_SEGMENT_LENGTH * max_segments ? ];
> 	struct acpi_buffer buffer = {
> 		.length = ACPI_PATH_SEGMENT_LENGTH,
> 		.buffer = name,
> 	};
> 

Hi Joe,

I thought about using stack initially, but I too was not sure how big
the buffer size should be, and was a bit afraid of causing kernel stack
overflow potentially.  Since it is mainly used for error paths in ACPI
hotplug handlers, I do not think alloc/free can lead any performance
impact.

Thanks,
-Toshi


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