unnecessary timer interrupt of slab.c and bdi tasks when the systemis in sleep state

[Wu, Xia]

Hi,
I found some unnecessary timer interrupts when the system enter sleep state.
(1) /mm/slab.c
   cache_reap() clean up on allocated memory every 2s. If the system is in sleep state, the system is waked-up when this timer expires. In fact,
   there isn't more slabs to been cleaned up in sleep state. 
   
(2) /mm/backing-dev.c and /fs/fs-writeback.c
   fs-writeback.c and backing-dev.c define a set of bdi tasks which write back dirty date to the file-system. These tasks typically runs 
   every 5 seconds even there isn't any dirty date in memory. These tasks call schedule_timeout () and schedule_timeout_interruptible().
   In fact, there isn't any dirty data in memory when the system is in sleep state. 

I think these timers should not wake-up the system when the system is in sleep state. In these cases, these timers only waste the CPU resource
and consume more power.

Regards
xia

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