Messages in this thread | | | Date | Thu, 03 Sep 2009 12:50:12 +0200 | From | Norbert van Bolhuis <> | Subject | PROBLEM: CONFIG_NO_HZ could cause software timeouts |
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The problem occurs when e.g. drivers use time_after(jiffes, timeout).
CONFIG_NO_HZ could make jiffies advance by more than 1. This is done by: tick_nohz_update_jiffies->tick_do_update_jiffies64->do_timer
If drivers use a timeout value of jiffies+1, "time_after(jiffies, timeout)" will be true after 1 interrupt (given that it advances jiffies by at least 2).
This is exactly what happens in cfi_cmdset_0002.c:do_write_buffer for our case (Powerpc MPC8313, linux-2.6.28, CONFIG_HZ=250, CONFIG_NO_HZ=y).
do_write_buffer does the following: unsigned long uWriteTimeout = ( HZ / 1000 ) + 1; ... timeo = jiffies + uWriteTimeout; ... for (;;) { ... if (time_after(jiffies, timeo) && !chip_ready(map, adr)) break; if (chip_ready(map, adr)) { xip_enable(map, chip, adr); goto op_done; } UDELAY(map, chip, adr, 1); } /* software timeout */ ret = -EIO; opdone: ...
I've seen a few software timeouts after the for-loop looped only 13 times (= 13 us delay, i.s.o. the expected 1 ms). Typically our NOR flash (S29GL01GP) may need upto ~ 200 us to be ready.
disabling CONFIG_NO_HZ fixes the problem. replacing time_after by a for-loop counter to loop max 1000 times also fixes the problem.
the latest kernel seems to have the same problem.
do I miss something here or is this a known problem of CONFIG_NO_HZ ?
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