Squashfs w/ xz compression on non-preemptable non-SMP linux kernel

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Squashfs w/ xz compression on non-preemptable non-SMP linux kernel

Klinedinst, Evan

Hi,

 

This is my first post to the list, so go easy on me lol ;).

 

Configuration:

-          4.4.6 based linux kernel

-          Non-SMP MIPS CPU

-          Preemption is disabled in the kernel (CONFIG_PREEMPTION=n)

-          JFFS2 filesystem – using squashfs - initrd with xz compression

-          mksquashfs root/ myTarget.initrd -noappend -always-use-fragments -b 1M -comp xz

 

Issue:

-          Whenever a user-space thread (real-time or not) invokes ‘popen (“someCommandA”, “r”)’, other user-space threads (real-time or not) get starved out for various ranges of time (upwards of ~350ms).

-          Using ‘perf timechart record sleep 10’ – I discovered that the ~350ms delay is coming from ‘someCommandA’ being decompressed.  If ‘someCommandA’ is run again then there is no noticeable delay because ‘someCommandA’ is now cached.

-          Running ‘echo 3 > /proc/sys/vm/drop_caches’ – will cause the delay again when invoking the same popen() call.

 

System:

-          ‘uname -a’ - Linux myTarget 4.4.6-at1 #2 Mon May 15 19:58:13 UTC 2017 mips GNU/Linux

 

Patch:

-          I’ve attached a patch (squashfs-xz.patch) against the 4.4.6 kernel that fixes the issue by adding a single cond_resched() call.

-          I’ve no idea if this is a proper place for a scheduler call, maybe more are needed elsewhere, and if there are alternatives in helping the decompression play nice with user-space threads.

-          If the kernel is compiled with preemption enabled (CONFIG_PREEMPTION=y) – the issue goes away entirely in our environment.

 

Concerns:

-          The ~350ms thread starvation delays are too long for our environment.

-          I’m afraid that if larger executables need to get decompressed, the thread starvation delays will just get much worse.

 

Thanks in advance,

Evan K



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