ARM SoC launched with Linux support (Linux Devices) [LWN.net]

ARM SoC launched with Linux support (Linux Devices)

Posted Jan 9, 2009 15:17 UTC (Fri) by simlo (subscriber, #10866)
In reply to: ARM SoC launched with Linux support (Linux Devices) by tialaramex
Parent article: ARM SoC launched with Linux support (Linux Devices)

As you appearently understood from the idea, the timer should prevent the
user from getting unlimited CPU. What should happen if he does anyway? Send him a SIGILL and let him deal with it.

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ARM SoC launched with Linux support (Linux Devices)

Posted Jan 9, 2009 15:45 UTC (Fri) by tialaramex (subscriber, #21167) [Link]

So, the user must absolutely avoid racing your timer, or he'll be punished with SIGILL. Thus the "feature" of disabling pre-emption becomes a thin wrapper around simple atomic ops only.

But, it incurs a full system call for every atomic op to let the kernel know that pre-emption can be re-enabled (if the user tries to avoid the system call, he's racing the timer and will SIGILL)

So now your solution has /worse/ performance than the existing solutions, and it's needlessly more complicated.

You might want to check the link nearby (about 0xffff0fc0) in this thread for what Linux actually provides to ARM users, and consider if your solution doesn't look a bit silly by comparison.