Hi. I noticed that the 'jiffies' variable has 'wall_jiffies + 1'
value in most of time. I'm using MIPS platform but I think this is
same for other platforms.
I suppose this is due to gcc does not know that jiffies_64 and jiffies
share same place.
In kernel/timer.c:
static inline void update_times(void)
{
unsigned long ticks;
ticks = jiffies - wall_jiffies;
if (ticks) {
wall_jiffies += ticks;
update_wall_time(ticks);
}
calc_load(ticks);
}
void do_timer(struct pt_regs *regs)
{
jiffies_64++;
update_times();
softlockup_tick(regs);
}
This is compiled MIPS code (gcc 3.4.5):
80056db4 <do_timer>:
...
80056de0: lui a3,0x8033
80056de4: lw v0,21400(a3) # load jiffies_64(lo)
80056de8: lui a1,0x8033
80056dec: lui t1,0x8033
80056df0: lw a2,21400(a1) # load jiffies
80056df4: lw v1,21404(a3) # load jiffies_64(hi)
80056df8: addiu v0,v0,1 # inc jiffies_64(lo)
80056dfc: lw t0,21360(t1) # load wall_jiffies
80056e00: sltiu a1,v0,1 # calc carry
80056e04: addu v1,v1,a1 # add carry to jiffies_64(hi)
80056e08: subu s4,a2,t0 # calc ticks (jiffies - wall_jiffies)
80056e0c: sw v0,21400(a3) # store jiffies_64(lo)
80056e10: sw v1,21404(a3) # store jiffies_64(hi)
80056e14: beqz s4,80057060 <do_timer+0x2ac>
80056e18: move s8,a0
The 'tick' variable is calculated using 'jiffies' value before
incrementing 'jiffies_64'. As a result, wall_jiffies will always one
smaller then jiffies on elsewhere.
I also checked x86 code (3.4.4).
c012696a <do_timer>:
...
c012696e: mov 0xc0482400,%eax # load jiffies
c0126973: addl $0x1,0xc0482400 # inc jiffies_64(lo)
c012697a: mov 0xc041a230,%edx # load wall_jiffies
c0126980: mov %eax,%ebx
c0126982: adcl $0x0,0xc0482404 # add carry to jiffies_64(hi)
c0126989: sub %edx,%ebx # calc ticks (jiffies - wall_jiffies)
c012698b: jne c01269a0 <do_timer+0x36>
Though I'm not familiar with x86, it looks same.
Is this really expected code? If no, how it can be fixed? Insert
"barrier()" right after "jiffies_64++" ?
---
Atsushi Nemoto
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