linux-kernel - Re: sched_clock

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Date:	Thu, 15 Apr 2010 18:32:57 +0200
From:	Michal Simek <michal.simek@...alogix.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Wu Zhangjin <wuzhangjin@...il.com>
Subject: Re: sched_clock - microblaze

Steven Rostedt wrote:
> On Thu, 2010-04-15 at 16:55 +0200, Michal Simek wrote:
>> Hi Thomas and Steven,
>>
>> I would like to improve time measuring for ftrace (Log below)
>>
>> I looked at http://lkml.org/lkml/2009/11/16/181
>> where Thomas suggested to create sched_clock() function.
>> I used the same solution as Wu proposed but it is not nice.
>>
>> Is unimplemented sched_clock the reason why ftrace not show fine grain time?
> 
> Yeah, sched_clock is used by ftrace for timings, so if it only returns
> jiffies, then that will, unfortunately, be the resolution of the tracer.
> 
> I've been told that if you make a higher resolution timer for sched
> clock, it will improve the scheduling in CFS.

We have two timers (in one IP).
timer0 - in IRQ mode - clock event
timer1 - free running up counter without IRQ - clock source

I hope that I can use timer1 in sched_clock too.

Anyway. I played with it a little bit and I also implemented microblaze 
specific sched_clock function (inspired by arch/arm/mach-ixp4xx/common.c)

unsigned long long notrace sched_clock(void)
{
	printk("%s\n", __func__);
	cycle_t cyc = microblaze_read(NULL);
	struct clocksource *cs = &clocksource_microblaze;
	return clocksource_cyc2ns(cyc, cs->mult, cs->shift);
}

(And comment sched_clock in kernel/sched_clock.c because our toolchain 
has the problem with weak symbols)

Recompile the kernel and then show the log_buf

<4>Ramdisk addr 0x00000003, Compiled-in FDT at 0xc0242a28
<5>Linux version 2.6.34-rc4-00053-gbc6ce8a-dirty (monstr@...str.eu) (gcc 
version 4.1.2) #27 Thu Apr 15 16:38:16 CEST 2010
<6>setup_cpuinfo: initialising
<6>setup_cpuinfo: Using full CPU PVR support
<6>cache: wb_msr
<6>setup_memory: Main mem: 0x48000000-0x50000000, size 0x08000000, 
klimit 0xc09c7000
<6>setup_memory: max_mapnr: 0x8000
<6>setup_memory: min_low_pfn: 0x48000
<6>setup_memory: max_low_pfn: 0x50000
<4>reserved 0 - 0x48000000-0x009c8000
<4>reserved 1 - 0x48fe0000-0x00020000
<4>reserved 2 - 0x4fffd8c8-0x00002738
<7>On node 0 totalpages: 32768
<7>free_area_init_node: node 0, pgdat c02bc70c, node_mem_map c09c8000
<7>  Normal zone: 256 pages used for memmap
<7>  Normal zone: 32512 pages, LIFO batch:7
<4>Built 1 zonelists in Zone order, mobility grouping on.  Total pages: 
32512
<5>Kernel command line:  console=ttyS0,115200
<6>PID hash table entries: 512 (order: -1, 2048 bytes)
<6>Dentry cache hash table entries: 16384 (order: 4, 65536 bytes)
<6>Inode-cache hash table entries: 8192 (order: 3, 32768 bytes)
<6>Memory: 119796k/131072k available
<4>sched_clock

The problem I see is that scheck_clock is used before our timer 
subsystem is initialized.
What am I missing?

Thanks,
Michal

-- 
Michal Simek, Ing. (M.Eng)
PetaLogix - Linux Solutions for a Reconfigurable World
w: www.petalogix.com p: +61-7-30090663,+42-0-721842854 f: +61-7-30090663
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