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Distributed Process Scheduling Algorithm
by Nitin Varyani Feb. 19, 2016

Feb. 19, 2016

Hi, I am given a task to design a distributed process scheduling algorithm. Current distributed OS are patch work over the linux kernels, that is, they are responsible for load balancing through process migration but the scheduling is taken care by the single machine linux kernels. My task is to make the scheduling algorithm itself as distributed. That is a scheduler itself makes a decision whether to migrate a task or to keep the task in the current system. I need some design aspects of how to achieve it. Another thing which I want to know is that whether this job is possible for a kernel newbie like me. Need urgent help. Nitin

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Difference dma_alloc_coherent() in x86_32 and x86_64
by tochansky＠tochlab.net Feb. 19, 2016

Feb. 19, 2016

Hello! I have a driver for PCI device which uses CMA framework for allocating big coherent blocks of memory for DMA. Allocation looks like: typedef struct { struct list_head list; uint32_t size8; void *kaddr; dma_addr_t paddr; } dma_region_t; LIST_HEAD(region_list); ...... void* AllocDMA( size_t size ) { dma_region_t *new_region; new_region = kmalloc(sizeof(dma_region_t), GFP_KERNEL); new_region->size8 = size; new_region->kaddr = dma_alloc_coherent( NULL, size, &new_region->paddr, GFP_KERNEL | GFP_DMA32 ); list_add(&new_region->list, &region_list); printk("pcidev: cma_alloc paddr %pad kaddr %p size %d\n", &new_region->paddr, new_region->kaddr, new_region->size8); return new_region->kaddr; } It works fine on kernel 3.18.26 in 32bit mode. When I reconfigure same kernel to run in 64bit mode(enabling in 'menuconfig' option '64-bit kernel') and trying to use this driver with it allocation failed with message in dmesg: ... [ 1393.835535] fallback device: swiotlb buffer is full (sz: 8388608 bytes) [ 1393.835579] pcidev: cma_alloc paddr 0xffff880234861220 kaddr (null) size 8388608 ... My kernel command line is: swiotlb=16384 iommu=soft cma=256M Anyone can explain this strange behavior? -- D

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Help about Ctrl-Z and Ctrl-Y(urgent)
by 慕冬亮 Feb. 19, 2016

Feb. 19, 2016

Hi everyone, Unix systems support a large number of special characters that receive special processing by the OS. One such character is the “suspend” character (ctrl-Z by default), that causes a SIGTSTP signal to be sent to the application process. The default action for the signal is to suspend execution of the process. The system responds to the suspend character (by sending the signal) as soon as possible after it is typed in. In older Unix systems there was another special character known as the “delayed suspend” character (ctrl-Y by default) that had the same effect as the suspend character, except that the signal is sent when the application process consumes the character, rather than right away. (That this feature is no longer supported is perhaps some indication of how useful it was ...) The suspend and the delayed-suspend characters have the effect, when they are processed, of deleting all characters currently waiting to be read by the user application that arrived before them. Explain, how both special characters are implemented? -- My best regards to you. No System Is Safe! mudongliang

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Question regarding upstreaming patches written by multiple people and checkpatch
by Joseph Bisch Feb. 18, 2016

Feb. 18, 2016

Hi, I am trying to upstream a patch set that wasn't written by me. The original patch has a lot of warnings and errors reported by checkpatch. Someone else corrected some of those with another patch and I added on a third patch correcting the rest of the warnings and errors that I thought were appropriate to fix. When checkpatch is run on the file that the patches affect, the output looks okay to me, but when I run checkpatch on the patches themselves, the patches still have the same errors and warnings, because I made my changes in a separate patch. So is it okay to have checkpatch errors/warnings for individual patches as long as the patch series as a whole is reasonably checkpatch clean or should I be amending my changes to the original commit and squashing the other cleanup patch into the first patch, so that I am left with one patch that passes checkpatch? If I should be squashing the patches into a single patch, how do I handle the signed-off-by lines as each of the three patches is currently signed off by the respective author of the patch? Obviously I can sign off on the squashed patch for myself. Do I just send it to the mailing list with just my sign off and CC the authors, so that they can sign off on it? From what I've been told, it sounds like CC'ing the authors and having them reply with their sign off is the appropriate way to handle this, but I want to make sure before I start mailing out patches, and this isn't something that I have been able to find an answer to. Cheers, Joseph

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Walk list of processes attached to a shared page
by W. Michael Petullo Feb. 18, 2016

Feb. 18, 2016

Is it possible to walk the processes already attached to a shared page in an implementation of security_shm_shmat()? I have a function: static int my_shm_shmat(struct shmid_kernel *shp, char __user *shmaddr, int shmflag) and I would like to find the processes already attached. It seems like rmap_walk() would help, but I cannot figure out how to get an appropriate struct page out of a struct shmid_kernel or unmapped address (i.e., shmaddr above). -- Mike :wq

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Re: Process scheduling
by Nitin Varyani Feb. 17, 2016

Feb. 17, 2016

On Mon, Feb 15, 2016 at 6:06 PM, Nitin Varyani <varyani.nitin1(a)gmail.com> wrote: > Hi > I have studied LInux kernel CFS scheduling algorithm - the > vruntime, weights, nice value, etc. I am able to understand the code. > Actually the task given to me is really very huge. I am told to design a > distributed process scheduling algorithm. A very simple implementation of > it will be sufficient for me. Current distributed OS are patch work over > the linux kernels, that is, they are responsible for load balancing through > process migration but the scheduling is taken care by the single machine > linux kernels. My task is to make the scheduling algorithm itself as > distributed. That is a scheduler makes a decision whether to migrate a task > or to keep the task in the current system. I need some design aspects of > how to achieve it. Another thing which I want to know is that whether this > job is possible for a kernel newbie like me. > > On Sat, Feb 13, 2016 at 3:12 PM, Nitin Varyani <varyani.nitin1(a)gmail.com> > wrote: > >> thanks >> >> On Sat, Feb 13, 2016 at 2:19 PM, Henrik Austad <henrik(a)austad.us> wrote: >> >>> On Sat, Feb 13, 2016 at 11:42:57AM +0530, Nitin Varyani wrote: >>> > Hello, >>> >>> Hi Nitin, >>> >>> > I want to understand the flow of code of process scheduler of >>> > linux kernel. What I have understood is that >>> > The task marks itself as sleeping, >>> > puts itself on a wait queue, >>> > removes itself from the red-black tree of runnable, and >>> > calls schedule() to select a new process to execute. >>> > >>> > for Waking back up >>> > The task is set as runnable, >>> > removed from the wait queue, >>> > and added back to the red-black tree. >>> > >>> > Can I get the details of which function does what? in sched/core.c and >>> in >>> > sched/fair.c >>> > I am concerned only with fair scheduler. There are so many functions in >>> > these two files that I am totally confused. >>> >>> Then core.c and fair.c is the best bet. >>> >>> You could also pick up a copy of Linux kernel development (By Love), it >>> gives a nice introduction to the overall flow of .. well mostly >>> everything. >>> :) >>> >>> In kernel/sched/sched.h you have a struct called 'struct sched_class" >>> which >>> is a set of function-points. This is used by the core machinery to call >>> into scheduling-class specific code. At the bottom of fair.c, you see >>> said >>> struct being populated. >>> >>> Also, if you want to see what really happens, try enabling >>> function-tracing, but limit it to sched-functions only (and sched-events, >>> those are also useful to see what triggers things) >>> >>> mount -t debugfs nodev /sys/kernel/debug >>> cd /sys/kernel/debug/tracing >>> echo 0 > tracing_on >>> echo function > current_tracer >>> echo "sched*" > set_ftrace_filter >>> echo 1 > events/sched/enable >>> echo 1 > tracing_on >>> ... wait for a few secs >>> echo 0 > tracing_on >>> >>> cat trace > /tmp/trace.txt >>> >>> Now, look at trace.txt and correlate it to the scheduler code :) >>> >>> Good luck! >>> >>> -- >>> Henrik Austad >>> >> >> >

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Patch Series
by Roger H Newell Feb. 17, 2016

Feb. 17, 2016

Hi all: I have a question about creating a patch series. I'm working in drivers/staging/wilc1000 fixing code style. If for example 2 errors are reported by checkpatch.pl error A and error B, it seems logical that I could fix all occurrences of error A as one patch and all occurrences of error B in a second patch. My question is, is it OK to fix all occurrences of error A across the files inside drivers/staging/wilc1000 as one patch and all occurrences of error B across all files inside drivers/staging/wilc1000 as a second patch? The alternative would be to create a patch that fixes all errors of A and a patch that fixes all errors of B per file ? In some cases there are so many occurrences of a particular error across files it probably does make more sense to create the patches per file. Any thoughts or clarifications on this ? Cheers: Roger H. Newell

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Finding when a chip was supported in the kernel
by Rob Groner Feb. 16, 2016

Feb. 16, 2016

Is there some specific place/way I can find the specific kernel version when support for an arbitrary chip was added? An example is the Intel 82574 Ethernet chip. I'm finding that google is failing me for finding the specific kernel of inclusion. This comes up somewhat often here at work (having to determine how far back our Linux support goes), so I'm trying to find the best way to determine this. Thank you. Rob Groner

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Merge conflict
by YU Bo Feb. 15, 2016

Feb. 15, 2016

Hi, Some days ago i followed the tips: http://kernelnewbies.org/FirstKernelPatch git clone greg's git tree and built branch first-patch. Due to my first patch that fixes coding style reported by checkptch.pl didn't applied,so i intend to rebase my git branch first-patch.The question i encounter is: ==================git command && output======== yubo@debian:~/maintree/staging$ git rebase first-patch It looks like git-am is in progress. Cannot rebase. yubo@debian:~/maintree/staging$ git am --abort You seem to have moved HEAD since the last 'am' failure. Not rewinding to ORIG_HEAD yubo@debian:~/maintree/staging$ git rebase first-patch First, rewinding head to replay your work on top of it... Applying: ovl: default permissions Using index info to reconstruct a base tree... Falling back to patching base and 3-way merge... ---- Auto-merging drivers/gpu/drm/i915/intel_display.c CONFLICT (content): Merge conflict in drivers/gpu/drm/i915/intel_display.c Failed to merge in the changes. Patch failed at 0010 drm/i915: Only run commit when crtc is active, v2. Is there something with wrong? Thanks in advance for the noisy. -- Best Regards

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counting semaphore question
by Shiva Shankar Feb. 15, 2016

Feb. 15, 2016

Hi all, Can you anyone tell me what is the exact use of counting semaphore in kernel ? When should we use it ? what is the practical usage of counting semaphores in Linux kernel ? I have seen only mutex (Binary semaphore)is used for synchronization not the counting semaphore(count >1). Any sugguestions would be appreciated Thanks in advance.

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