On Sun, Dec 16, 2018 at 09:27:46AM +0100, Greg KH wrote:
On Sun, Dec 16, 2018 at 02:21:55PM +0800, Barry Kauler wrote:
Mr Gleixner, I was upset when I compiled 4.14.87 found that SCHED_SMT had been forced on. At the time, I just reported it to my blog, and posted a question about it to a couple of forums, and stayed with an earlier kernel.
However, when 4.14.88 came out, and still the same situation, alarm bells went off, and I looked through the kernel changelog. Found it, 4.14.86:
"x86/Kconfig: Select SCHED_SMT if SMP enabled"
Then:
"CONFIG_SCHED_SMT is enabled by all distros, so there is not a real point to have it configurable. ..."
...that is a lie. It would be correct to state that is true of the distros you use, and presumably also for all of you guys who signed off on it.
Puppy Linux is an example of a distro that has mostly not had SCHED_SMT enabled. Ditto for most of the forks of Puppy. Two distros that I currently maintain, Quirky and EasyOS (easyos.org) have SMP enabled but not SCHED_SMT.
The difference between them is important, they should remain independently settable. I am so surprised that all of you guys went along with forcing it on.
Is having this option enabled causing a problem for you? Does it cause slowdowns or runtime issues?
From the blog:
Note, way back in the early days when I was first compiling the kernel for multi-core CPUs, SMT (hyperthreading), which simulates more than one core for each actual core, was causing file corruption. So, disabled it, and never trusted it after that. <<
Since nearly everyone else has the option enabled we are quite confident there is no data corruption caused by it. So there shouldn't be any reason to turn it off.
-Andi