CODEWITHSUNDEEP
linux-kernel
David Poole
David Poole

Reputation: 3510

Where is jiffies computed in the Linux kernel?

I'm porting 3.14 to an ARM-based SOC which was successfully running the 3.2 kernel.

I'm getting stuck in the code that calibrates jiffies.

calibrate_delay_converge()-init/calibrate.c

    /* wait for "start of" clock tick */
    ticks = jiffies;
    while (ticks == jiffies)   <---- infinite loop waiting for jiffies to change
            ; /* nothing */
    /* Go .. */

jiffies isn't being updated. Where does jiffies get updated? I'm looking for smoking gun code like jiffies++ or a .S file updating jiffies.

I went down the rabbit hole of the timers and interrupt system in Linux. The timer interrupt isn't enabled (in the PL190 HW). I'm hoping if I can track from the bottom up (where jiffies SHOULD be called), I can find why the interrupt(s) aren't being enabled.

Upvotes: 3

Views: 3403

Answers (1)

mrjoltcola
mrjoltcola

Reputation: 20842

Look at do_timer(). It was moved to kernel/time/timekeeping.c at some point in the past few years.

jiffies does not directly get incremented, it gets assigned the low order 32-bit of jiffies_64

/* 
 * The 64-bit jiffies value is not atomic - you MUST NOT read it 
 * without sampling the sequence number in xtime_lock. 
 * jiffies is defined in the linker script... 
 */ 
void do_timer(unsigned long ticks) 
{ 
 jiffies_64 += ticks; 
 update_wall_time(); 
 calc_global_load(ticks); 
}

In 3.2 it is http://lxr.free-electrons.com/source/kernel/time/timekeeping.c?v=3.2#L1192

jiffies gets the value from jiffies_64 here in the machine specific file:

http://lxr.free-electrons.com/source/arch/arm/kernel/vmlinux.lds.S?v=3.2

 36 #ifndef __ARMEB__
 37 jiffies = jiffies_64;
 38 #else
 39 jiffies = jiffies_64 + 4;
 40 #endif

Upvotes: 7

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