fs/buffer.c: disable per-CPU buffer_head cache for isolated CPUs

For certain types of applications (for example PLC software or
RAN processing), upon occurrence of an event, it is necessary to
complete a certain task in a maximum amount of time (deadline).

One way to express this requirement is with a pair of numbers,
deadline time and execution time, where:

        * deadline time: length of time between event and deadline.
        * execution time: length of time it takes for processing of event
                          to occur on a particular hardware platform
                          (uninterrupted).

The particular values depend on use-case. For the case
where the realtime application executes in a virtualized
guest, an IPI which must be serviced in the host will cause
the following sequence of events:

        1) VM-exit
        2) execution of IPI (and function call)
        3) VM-entry

Which causes an excess of 50us latency as observed by cyclictest
(this violates the latency requirement of vRAN application with 1ms TTI,
for example).

invalidate_bh_lrus calls an IPI on each CPU that has non empty
per-CPU cache:

        on_each_cpu_cond(has_bh_in_lru, invalidate_bh_lru, NULL, 1);

The performance when using the per-CPU LRU cache is as follows:

 42 ns per __find_get_block
 68 ns per __find_get_block_slow

Given that the main use cases for latency sensitive applications
do not involve block I/O (data necessary for program operation is
locked in RAM), disable per-CPU buffer_head caches for isolated CPUs.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Acked-by: Frederic Weisbecker <frederic@kernel.org>
Message-Id: <ZJtBrybavtb1x45V@tpad>
Signed-off-by: Christian Brauner <brauner@kernel.org>