5c63db59c5
A large part of the kernel boot time is creating the kernel linear map
page tables. When rodata=full, all memory is mapped by pte. And when
there is lots of physical ram, there are lots of pte tables to populate.
The primary cost associated with this is mapping and unmapping the pte
table memory in the fixmap; at unmap time, the TLB entry must be
invalidated and this is expensive.
Previously, each pmd and pte table was fixmapped/fixunmapped for each
cont(pte|pmd) block of mappings (16 entries with 4K granule). This means
we ended up issuing 32 TLBIs per (pmd|pte) table during the population
phase.
Let's fix that, and fixmap/fixunmap each page once per population, for a
saving of 31 TLBIs per (pmd|pte) table. This gives a significant boot
speedup.
Execution time of map_mem(), which creates the kernel linear map page
tables, was measured on different machines with different RAM configs:
| Apple M2 VM | Ampere Altra| Ampere Altra| Ampere Altra
| VM, 16G | VM, 64G | VM, 256G | Metal, 512G
---------------|-------------|-------------|-------------|-------------
| ms (%) | ms (%) | ms (%) | ms (%)
---------------|-------------|-------------|-------------|-------------
before | 168 (0%) | 2198 (0%) | 8644 (0%) | 17447 (0%)
after | 78 (-53%) | 435 (-80%) | 1723 (-80%) | 3779 (-78%)
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Tested-by: Itaru Kitayama <itaru.kitayama@fujitsu.com>
Tested-by: Eric Chanudet <echanude@redhat.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240412131908.433043-2-ryan.roberts@arm.com
Signed-off-by: Will Deacon <will@kernel.org>
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 reStructuredText 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.