• Venkatesh Pallipadi's avatar
    [PATCH] Platform SMIs and their interferance with tsc based delay calibration · 8a9e1b0f
    Venkatesh Pallipadi authored
    Issue:
    Current tsc based delay_calibration can result in significant errors in
    loops_per_jiffy count when the platform events like SMIs
    (System Management Interrupts that are non-maskable) are present. This could
    lead to potential kernel panic(). This issue is becoming more visible with 2.6
    kernel (as default HZ is 1000) and on platforms with higher SMI handling
    latencies. During the boot time, SMIs are mostly used by BIOS (for things
    like legacy keyboard emulation).
    
    Description:
    The psuedocode for current delay calibration with tsc based delay looks like
    (0) Estimate a value for loops_per_jiffy
    (1) While (loops_per_jiffy estimate is accurate enough)
    (2)   wait for jiffy transition (jiffy1)
    (3)   Note down current tsc (tsc1)
    (4)   loop until tsc becomes tsc1 + loops_per_jiffy
    (5)   check whether jiffy changed since jiffy1 or not and refine
    loops_per_jiffy estimate
    
    Consider the following cases
    Case 1:
    If SMIs happen between (2) and (3) above, we can end up with a
    loops_per_jiffy value that is too low. This results in shorted delays and
    kernel can panic () during boot (Mostly at IOAPIC timer initialization
    timer_irq_works() as we don't have enough timer interrupts in a specified
    interval).
    
    Case 2:
    If SMIs happen between (3) and (4) above, then we can end up with a
    loops_per_jiffy value that is too high. And with current i386 code, too
    high lpj value (greater than 17M) can result in a overflow in
    delay.c:__const_udelay() again resulting in shorter delay and panic().
    
    Solution:
    The patch below makes the calibration routine aware of asynchronous events
    like SMIs. We increase the delay calibration time and also identify any
    significant errors (greater than 12.5%) in the calibration and notify it to
    user.
    
    Patch below changes both i386 and x86-64 architectures to use this
    new and improved calibrate_delay_direct() routine.
    Signed-off-by: default avatarVenkatesh Pallipadi <venkatesh.pallipadi@intel.com>
    Signed-off-by: default avatarAdrian Bunk <bunk@stusta.de>
    Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
    8a9e1b0f
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