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ec79315589
Collecting crdump involves reading vsc registers from pci config space
of mlx device, which can take long time to complete. This might result
in starving other threads waiting to run on the cpu.
Numbers I got from testing ConnectX-5 Ex MCX516A-CDAT in the lab:
- mlx5_vsc_gw_read_block_fast() was called with length = 1310716.
- mlx5_vsc_gw_read_fast() reads 4 bytes at a time. It was not used to
read the entire 1310716 bytes. It was called 53813 times because
there are jumps in read_addr.
- On average mlx5_vsc_gw_read_fast() took 35284.4ns.
- In total mlx5_vsc_wait_on_flag() called vsc_read() 54707 times.
The average time for each call was 17548.3ns. In some instances
vsc_read() was called more than one time when the flag was not set.
As expected the thread released the cpu after 16 iterations in
mlx5_vsc_wait_on_flag().
- Total time to read crdump was 35284.4ns * 53813 ~= 1.898s.
It was seen in the field that crdump can take more than 5 seconds to
complete. During that time mlx5_vsc_wait_on_flag() did not release the
cpu because it did not complete 16 iterations. It is believed that pci
config reads were slow. Adding cond_resched() every 128 register read
improves the situation. In the common case the, crdump takes ~1.8989s,
the thread yields the cpu every ~4.51ms. If crdump takes ~5s, the thread
yields the cpu every ~18.0ms.
Fixes:
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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.