e2768b798a
In preparation for adding support for LPA2 to the tlb invalidation routines, modify the algorithm used by range-based tlbi to start at the highest 'scale' and decrement instead of starting at the lowest 'scale' and incrementing. This new approach makes it possible to maintain 64K alignment as we work through the range, until the last op (at scale=0). This is required when LPA2 is enabled. (This part will be added in a subsequent commit). This change is separated into its own patch because it will also impact non-LPA2 systems, and I want to make it easy to bisect in case it leads to performance regression (see below for benchmarks that suggest this should not be a problem). The original commit (d1d3aa98 "arm64: tlb: Use the TLBI RANGE feature in arm64") stated this as the reason for _incrementing_ scale: However, in most scenarios, the pages = 1 when flush_tlb_range() is called. Start from scale = 3 or other proper value (such as scale =ilog2(pages)), will incur extra overhead. So increase 'scale' from 0 to maximum. But pages=1 is already special cased by the non-range invalidation path, which will take care of it the first time through the loop (both in the original commit and in my change), so I don't think switching to decrement scale should have any extra performance impact after all. Indeed benchmarking kernel compilation, a TLBI-heavy workload, suggests that this new approach actually _improves_ performance slightly (using a virtual machine on Apple M2): Table shows time to execute kernel compilation workload with 8 jobs, relative to baseline without this patch (more negative number is bigger speedup). Repeated 9 times across 3 system reboots: | counter | mean | stdev | |:----------|-----------:|----------:| | real-time | -0.6% | 0.0% | | kern-time | -1.6% | 0.5% | | user-time | -0.4% | 0.1% | Reviewed-by: Oliver Upton <oliver.upton@linux.dev> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> Signed-off-by: Marc Zyngier <maz@kernel.org> Link: https://lore.kernel.org/r/20231127111737.1897081-2-ryan.roberts@arm.com
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.