Merge remote-tracking branch 'fscache/fscache'

[karo-tx-linux.git] / Documentation / kernel-per-CPU-kthreads.txt

diff --git a/Documentation/kernel-per-CPU-kthreads.txt b/Documentation/kernel-per-CPU-kthreads.txt
index cbf7ae412da4ab915a345582cf6b5fc93ca00ed5..32351bfabf2038617c35326f4545ac9f33b02073 100644 (file)
--- a/Documentation/kernel-per-CPU-kthreads.txt
+++ b/Documentation/kernel-per-CPU-kthreads.txt
@@ -157,6 +157,53 @@ RCU_SOFTIRQ:  Do at least one of the following:
                calls and by forcing both kernel threads and interrupts
                to execute elsewhere.
 
+Name: kworker/%u:%d%s (cpu, id, priority)
+Purpose: Execute workqueue requests
+To reduce its OS jitter, do any of the following:
+1.     Run your workload at a real-time priority, which will allow
+       preempting the kworker daemons.
+2.     Do any of the following needed to avoid jitter that your
+       application cannot tolerate:
+       a.      Build your kernel with CONFIG_SLUB=y rather than
+               CONFIG_SLAB=y, thus avoiding the slab allocator's periodic
+               use of each CPU's workqueues to run its cache_reap()
+               function.
+       b.      Avoid using oprofile, thus avoiding OS jitter from
+               wq_sync_buffer().
+       c.      Limit your CPU frequency so that a CPU-frequency
+               governor is not required, possibly enlisting the aid of
+               special heatsinks or other cooling technologies.  If done
+               correctly, and if you CPU architecture permits, you should
+               be able to build your kernel with CONFIG_CPU_FREQ=n to
+               avoid the CPU-frequency governor periodically running
+               on each CPU, including cs_dbs_timer() and od_dbs_timer().
+               WARNING:  Please check your CPU specifications to
+               make sure that this is safe on your particular system.
+       d.      It is not possible to entirely get rid of OS jitter
+               from vmstat_update() on CONFIG_SMP=y systems, but you
+               can decrease its frequency by writing a large value to
+               /proc/sys/vm/stat_interval.  The default value is HZ,
+               for an interval of one second.  Of course, larger values
+               will make your virtual-memory statistics update more
+               slowly.  Of course, you can also run your workload at
+               a real-time priority, thus preempting vmstat_update().
+       e.      If running on high-end powerpc servers, build with
+               CONFIG_PPC_RTAS_DAEMON=n.  This prevents the RTAS
+               daemon from running on each CPU every second or so.
+               (This will require editing Kconfig files and will defeat
+               this platform's RAS functionality.)  This avoids jitter
+               due to the rtas_event_scan() function.
+               WARNING:  Please check your CPU specifications to
+               make sure that this is safe on your particular system.
+       f.      If running on Cell Processor, build your kernel with
+               CBE_CPUFREQ_SPU_GOVERNOR=n to avoid OS jitter from
+               spu_gov_work().
+               WARNING:  Please check your CPU specifications to
+               make sure that this is safe on your particular system.
+       g.      If running on PowerMAC, build your kernel with
+               CONFIG_PMAC_RACKMETER=n to disable the CPU-meter,
+               avoiding OS jitter from rackmeter_do_timer().
+
 Name: rcuc/%u
 Purpose: Execute RCU callbacks in CONFIG_RCU_BOOST=y kernels.
 To reduce its OS jitter, do at least one of the following:
@@ -185,7 +232,7 @@ Purpose: Offload RCU callbacks from the corresponding CPU.
 To reduce its OS jitter, do at least one of the following:
 1.     Use affinity, cgroups, or other mechanism to force these kthreads
        to execute on some other CPU.
-2.     Build with CONFIG_RCU_NOCB_CPUS=n, which will prevent these
+2.     Build with CONFIG_RCU_NOCB_CPU=n, which will prevent these
        kthreads from being created in the first place.  However, please
        note that this will not eliminate OS jitter, but will instead
        shift it to RCU_SOFTIRQ.