tree 18623b21c6ba79b465ed80453ad33cbe1c12e95e
parent 6da4b3ab9a6e9b1b5f90322ab3fa3a7dd18edb19
author Long Li <longli@microsoft.com> 1541476800 +0000
committer Thomas Gleixner <tglx@linutronix.de> 1541542813 +0100

genirq/matrix: Improve target CPU selection for managed interrupts.

On large systems with multiple devices of the same class (e.g. NVMe disks,
using managed interrupts), the kernel can affinitize these interrupts to a
small subset of CPUs instead of spreading them out evenly.

irq_matrix_alloc_managed() tries to select the CPU in the supplied cpumask
of possible target CPUs which has the lowest number of interrupt vectors
allocated.

This is done by searching the CPU with the highest number of available
vectors. While this is correct for non-managed CPUs it can select the wrong
CPU for managed interrupts. Under certain constellations this results in
affinitizing the managed interrupts of several devices to a single CPU in
a set.

The book keeping of available vectors works the following way:

 1) Non-managed interrupts:

    available is decremented when the interrupt is actually requested by
    the device driver and a vector is assigned. It's incremented when the
    interrupt and the vector are freed.

 2) Managed interrupts:

    Managed interrupts guarantee vector reservation when the MSI/MSI-X
    functionality of a device is enabled, which is achieved by reserving
    vectors in the bitmaps of the possible target CPUs. This reservation
    decrements the available count on each possible target CPU.

    When the interrupt is requested by the device driver then a vector is
    allocated from the reserved region. The operation is reversed when the
    interrupt is freed by the device driver. Neither of these operations
    affect the available count.

    The reservation persist up to the point where the MSI/MSI-X
    functionality is disabled and only this operation increments the
    available count again.

For non-managed interrupts the available count is the correct selection
criterion because the guaranteed reservations need to be taken into
account. Using the allocated counter could lead to a failing allocation in
the following situation (total vector space of 10 assumed):

		 CPU0	CPU1
 available:	    2	   0
 allocated:	    5	   3   <--- CPU1 is selected, but available space = 0
 managed reserved:  3	   7

 while available yields the correct result.

For managed interrupts the available count is not the appropriate
selection criterion because as explained above the available count is not
affected by the actual vector allocation.

The following example illustrates that. Total vector space of 10
assumed. The starting point is:

		 CPU0	CPU1
 available:	    5	   4
 allocated:	    2	   3
 managed reserved:  3	   3

 Allocating vectors for three non-managed interrupts will result in
 affinitizing the first two to CPU0 and the third one to CPU1 because the
 available count is adjusted with each allocation:

		  CPU0	CPU1
 available:	     5	   4	<- Select CPU0 for 1st allocation
 --> allocated:	     3	   3

 available:	     4	   4	<- Select CPU0 for 2nd allocation
 --> allocated:	     4	   3

 available:	     3	   4	<- Select CPU1 for 3rd allocation
 --> allocated:	     4	   4

 But the allocation of three managed interrupts starting from the same
 point will affinitize all of them to CPU0 because the available count is
 not affected by the allocation (see above). So the end result is:

		  CPU0	CPU1
 available:	     5	   4
 allocated:	     5	   3

Introduce a "managed_allocated" field in struct cpumap to track the vector
allocation for managed interrupts separately. Use this information to
select the target CPU when a vector is allocated for a managed interrupt,
which results in more evenly distributed vector assignments. The above
example results in the following allocations:

		 CPU0	CPU1
 managed_allocated: 0	   0	<- Select CPU0 for 1st allocation
 --> allocated:	    3	   3

 managed_allocated: 1	   0	<- Select CPU1 for 2nd allocation
 --> allocated:	    3	   4

 managed_allocated: 1	   1	<- Select CPU0 for 3rd allocation
 --> allocated:	    4	   4

The allocation of non-managed interrupts is not affected by this change and
is still evaluating the available count.

The overall distribution of interrupt vectors for both types of interrupts
might still not be perfectly even depending on the number of non-managed
and managed interrupts in a system, but due to the reservation guarantee
for managed interrupts this cannot be avoided.

Expose the new field in debugfs as well.

[ tglx: Clarified the background of the problem in the changelog and
  	described it independent of NVME ]

Signed-off-by: Long Li <longli@microsoft.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Michael Kelley <mikelley@microsoft.com>
Link: https://lkml.kernel.org/r/20181106040000.27316-1-longli@linuxonhyperv.com